Add prospector for checking Python code (#1183)

doc/ale-python.txt

@@ -122,6 +122,46 @@ g:ale_python_mypy_use_global                     *g:ale_python_mypy_use_global*
   See |ale-integrations-local-executables|
 
 
+===============================================================================
+prospector                                              *ale-python-prospector*
+
+g:ale_python_prospector_executable               *g:ale_python_prospector_executable*
+                                                 *b:ale_python_prospector_executable*
+  Type: |String|
+  Default: `'prospector'`
+
+  See |ale-integrations-local-executables|
+
+
+g:ale_python_prospector_options                  *g:ale_python_prospector_options*
+                                                 *b:ale_python_prospector_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to add command-line arguments to the prospector
+  invocation.
+
+  For example, to dynamically switch between programs targeting Python 2 and
+  Python 3, you may want to set >
+
+  let g:ale_python_prospector_executable = 'python3'
+  " or 'python' for Python 2
+  let g:ale_python_prospector_options = '--rcfile /path/to/.prospector.yaml'
+  " The virtualenv detection needs to be disabled.
+  let g:ale_python_prospector_use_global = 0
+
+  after making sure it's installed for the appropriate Python versions (e.g.
+  `python3 -m pip install --user prospector`).
+
+
+g:ale_python_prospector_use_global               *g:ale_python_prospector_use_global*
+                                                 *b:ale_python_prospector_use_global*
+  Type: |Number|
+  Default: `0`
+
+  See |ale-integrations-local-executables|
+
+
 ===============================================================================
 pycodestyle                                            *ale-python-pycodestyle*