Fix #516 - Add support for pyflakes for Python

test/command_callback/test_pyflakes_command_callback.vader

@@ -0,0 +1,49 @@
+Before:
+  Save g:ale_python_pyflakes_executable
+  Save g:ale_python_pyflakes_use_global
+
+  unlet! g:ale_python_pyflakes_executable
+  unlet!  g:ale_python_pyflakes_use_global
+
+  let b:bin_dir = has('win32') ? 'Scripts' : 'bin'
+
+  call ale#test#SetDirectory('/testplugin/test/command_callback')
+
+  runtime ale_linters/python/pyflakes.vim
+
+After:
+  Restore
+
+  unlet! b:bin_dir
+  unlet! b:executable
+
+  call ale#test#RestoreDirectory()
+  call ale#linter#Reset()
+
+Execute(The pyflakes command callback should return default string):
+  AssertEqual ale#Escape('pyflakes') . ' %t',
+  \ ale_linters#python#pyflakes#GetCommand(bufnr(''))
+
+Execute(The pyflakes executable should be configurable):
+  let g:ale_python_pyflakes_executable = '~/.local/bin/pyflakes'
+
+  AssertEqual ale#Escape('~/.local/bin/pyflakes') . ' %t',
+  \ ale_linters#python#pyflakes#GetCommand(bufnr(''))
+
+Execute(The pyflakes executable should be run from the virtualenv path):
+  call ale#test#SetFilename('python_paths/with_virtualenv/subdir/foo/bar.py')
+
+  let b:executable = ale#path#Winify(
+  \ g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/pyflakes'
+  \)
+
+  AssertEqual ale#Escape(b:executable) . ' %t',
+  \ ale_linters#python#pyflakes#GetCommand(bufnr(''))
+
+Execute(You should be able to override the pyflakes virtualenv lookup):
+  call ale#test#SetFilename('python_paths/with_virtualenv/subdir/foo/bar.py')
+
+  let g:ale_python_pyflakes_use_global = 1
+
+  AssertEqual ale#Escape('pyflakes') . ' %t',
+  \ ale_linters#python#pyflakes#GetCommand(bufnr(''))