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.

ale_linters/python/mypy.vim

@@ -5,8 +5,13 @@ call ale#Set('python_mypy_executable', 'mypy')
 call ale#Set('python_mypy_ignore_invalid_syntax', 0)
 call ale#Set('python_mypy_options', '')
 call ale#Set('python_mypy_use_global', get(g:, 'ale_use_global_executables', 0))
+call ale#Set('python_mypy_auto_pipenv', 0)
 
 function! ale_linters#python#mypy#GetExecutable(buffer) abort
+    if ale#Var(a:buffer, 'python_mypy_auto_pipenv') && ale#python#PipenvPresent(a:buffer)
+        return 'pipenv'
+    endif
+
     return ale#python#FindExecutable(a:buffer, 'python_mypy', ['mypy'])
 endfunction