Add g:ale_python_flake8_executable (#174)

* Add g:ale_python_flake8_executable

Closes #172.

* Add g:ale_python_flake8_args

* Always add - to flake8 invocations

ale_linters/python/flake8.vim

@@ -1,9 +1,24 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: flake8 for python files
 
+let g:ale_python_flake8_executable =
+\   get(g:, 'ale_python_flake8_executable', 'flake8')
+
+let g:ale_python_flake8_args =
+\   get(g:, 'ale_python_flake8_args', '')
+
+function! ale_linters#python#flake8#GetExecutable(buffer) abort
+    return g:ale_python_flake8_executable
+endfunction
+
+function! ale_linters#python#flake8#GetCommand(buffer) abort
+    return ale_linters#python#flake8#GetExecutable(a:buffer)
+    \   . ' ' . g:ale_python_flake8_args . ' -'
+endfunction
+
 call ale#linter#Define('python', {
 \   'name': 'flake8',
-\   'executable': 'flake8',
-\   'command': 'flake8 -',
+\   'executable_callback': 'ale_linters#python#flake8#GetExecutable',
+\   'command_callback': 'ale_linters#python#flake8#GetCommand',
 \   'callback': 'ale#handlers#HandlePEP8Format',
 \})

doc/ale.txt

@@ -598,6 +598,34 @@ g:ale_c_clang_options                                   *g:ale_c_clang_options*
 
   This variable can be change to modify flags given to clang.
 
+-------------------------------------------------------------------------------
+4.17. python-flake8                          *ale-linter-options-python-flake8*
+
+g:ale_python_flake8_executable                 *g:ale_python_flake8_executable*
+
+  Type: |String|
+  Default: `'flake8'`
+
+  This variable can be changed to modify the executable used for flake8.
+
+
+g:ale_python_flake8_args                             *g:ale_python_flake8_args*
+
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to add command-line arguments to the flake8
+  invocation.
+
+For example, to dynamically switch between programs targeting Python 2 and
+Python 3, you may want to set >
+
+  let g:ale_python_flake8_executable = 'python3'   " or 'python' for Python 2
+  let g:ale_python_flake8_args = '-m flake8'
+
+after making sure it's installed for the appropriate Python versions (e.g.
+`python3 -m pip install --user flake8`).
+
 
 ===============================================================================
 5. Commands/Keybinds                                             *ale-commands*