Add support for pydocstyle linter (#2085)

The linter can correctly parse pydocstyle output with any of the following
command-line options enabled: --explain, --source, --debug, and/or
--verbose

test/command_callback/test_pydocstyle_command_callback.vader

@@ -0,0 +1,39 @@
+Before:
+  call ale#assert#SetUpLinterTest('python', 'pydocstyle')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The pydocstyle command callback should return default string):
+  AssertLinter 'pydocstyle',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('pydocstyle') . '  ' . ale#Escape('dummy.txt')
+
+Execute(The pydocstyle command callback should allow options):
+  let g:ale_python_pydocstyle_options = '--verbose'
+
+  AssertLinter 'pydocstyle',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('pydocstyle') . ' --verbose ' . ale#Escape('dummy.txt')
+
+Execute(The pydocstyle executable should be configurable):
+  let g:ale_python_pydocstyle_executable = '~/.local/bin/pydocstyle'
+
+  AssertLinter '~/.local/bin/pydocstyle',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('~/.local/bin/pydocstyle') . '  ' . ale#Escape('dummy.txt')
+
+Execute(Setting executable to 'pipenv' appends 'run pydocstyle'):
+  let g:ale_python_pydocstyle_executable = 'path/to/pipenv'
+
+  AssertLinter 'path/to/pipenv',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('path/to/pipenv') . ' run pydocstyle  ' . ale#Escape('dummy.txt')
+
+Execute(Pipenv is detected when python_pydocstyle_auto_pipenv is set):
+  let g:ale_python_pydocstyle_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('pipenv') . ' run pydocstyle  ' . ale#Escape('whatever.py')