Add python_[linter]_auto_pipenv options for python linters (fixes #1656)

When set to true, and the buffer is currently inside a pipenv,
GetExecutable will return "pipenv", which will trigger the existing
functionality to append the correct pipenv arguments to run each linter.

Defaults to false.

I was going to implement ale#python#PipenvPresent by invoking
`pipenv --venv` or `pipenv --where`, but it seemed to be abominably
slow, even to the point where the test suite wasn't even finishing
("Tried to run tests 3 times"). The diff is:

diff --git a/autoload/ale/python.vim b/autoload/ale/python.vim
index 7baae079..8c100d41 100644
--- a/autoload/ale/python.vim
+++ b/autoload/ale/python.vim
@@ -106,5 +106,9 @@ endfunction

" Detects whether a pipenv environment is present.
function! ale#python#PipenvPresent(buffer) abort
-    return findfile('Pipfile.lock', expand('#' . a:buffer . ':p:h') . ';') isnot# ''
+    let l:cd_string = ale#path#BufferCdString(a:buffer)
+    let l:output = systemlist(l:cd_string . 'pipenv --where')[0]
+    " `pipenv --where` returns the path to the dir containing the Pipfile
+    " if in a pipenv, or some error text otherwise.
+    return strpart(l:output, 0, 18) !=# "No Pipfile present"
 endfunction

Using vim's `findfile` is much faster, behaves correctly in the majority
of situations, and also works reliably when the `pipenv` command doesn't
exist.

doc/ale-python.txt

@@ -137,6 +137,15 @@ g:ale_python_flake8_use_global                 *g:ale_python_flake8_use_global*
   Both variables can be set with `b:` buffer variables instead.
 
 
+g:ale_python_flake8_auto_pipenv               *g:ale_python_flake8_auto_pipenv*
+                                              *b:ale_python_flake8_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 isort                                                        *ale-python-isort*
 
@@ -211,6 +220,15 @@ g:ale_python_mypy_use_global                     *g:ale_python_mypy_use_global*
   See |ale-integrations-local-executables|
 
 
+g:ale_python_mypy_auto_pipenv                   *g:ale_python_mypy_auto_pipenv*
+                                                *b:ale_python_mypy_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 prospector                                              *ale-python-prospector*
 
@@ -253,6 +271,15 @@ g:ale_python_prospector_use_global         *g:ale_python_prospector_use_global*
   See |ale-integrations-local-executables|
 
 
+g:ale_python_prospector_auto_pipenv       *g:ale_python_prospector_auto_pipenv*
+                                          *b:ale_python_prospector_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 pycodestyle                                            *ale-python-pycodestyle*
 
@@ -284,6 +311,15 @@ g:ale_python_pycodestyle_use_global       *g:ale_python_pycodestyle_use_global*
   See |ale-integrations-local-executables|
 
 
+g:ale_python_pycodestyle_auto_pipenv     *g:ale_python_pycodestyle_auto_pipenv*
+                                         *b:ale_python_pycodestyle_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 pyflakes                                                  *ale-python-pyflakes*
 
@@ -298,6 +334,15 @@ g:ale_python_pyflakes_executable             *g:ale_python_pyflakes_executable*
   Set this to `'pipenv'` to invoke `'pipenv` `run` `pyflakes'`.
 
 
+g:ale_python_pyflakes_auto_pipenv           *g:ale_python_pyflakes_auto_pipenv*
+                                            *b:ale_python_pyflakes_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 pylint                                                      *ale-python-pylint*
 
@@ -350,6 +395,15 @@ g:ale_python_pylint_use_global                 *g:ale_python_pylint_use_global*
   See |ale-integrations-local-executables|
 
 
+g:ale_python_pylint_auto_pipenv               *g:ale_python_pylint_auto_pipenv*
+                                              *b:ale_python_pylint_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 pyls                                                          *ale-python-pyls*
 
@@ -374,6 +428,15 @@ g:ale_python_pyls_use_global                     *g:ale_python_pyls_use_global*
   See |ale-integrations-local-executables|
 
 
+g:ale_python_pyls_auto_pipenv                   *g:ale_python_pyls_auto_pipenv*
+                                                *b:ale_python_pyls_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 pyre                                                          *ale-python-pyre*
 
@@ -398,6 +461,15 @@ g:ale_python_pyre_use_global                     *g:ale_python_pyre_use_global*
   See |ale-integrations-local-executables|
 
 
+g:ale_python_pyre_auto_pipenv                   *g:ale_python_pyre_auto_pipenv*
+                                                *b:ale_python_pyre_auto_pipenv*
+  Type: |Number|
+  Default: `0`
+
+  Detect whether the file is inside a pipenv, and set the executable to `pipenv`
+  if true. This is overridden by a manually-set executable.
+
+
 ===============================================================================
 vulture                                                    *ale-python-vulture*