Add python_auto_pipenv config var for all python linters.

This allows a user to set one variable instead of eight.

doc/ale-python.txt

@@ -2,6 +2,14 @@
 ALE Python Integration                                     *ale-python-options*
 
 
+g:ale_python_auto_pipenv                             *g:ale_python_auto_pipenv*
+                                                     *b:ale_python_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.
+
 ===============================================================================
 ALE Python Project Root Behavior                              *ale-python-root*