Add python mypy support

This adds support for the official optional python typechecker.

ale_linters/python/mypy.vim

@@ -0,0 +1,17 @@
+" Author: Keith Smiley <k@keith.so>
+" Description: mypy support for optional python typechecking
+
+let g:ale_python_mypy_args = get(g:, 'ale_python_mypy_args', '')
+
+function! g:ale_linters#python#mypy#GetCommand(buffer) abort
+    return g:ale#util#stdin_wrapper
+          \ . ' .py mypy --show-column-numbers '
+          \ . g:ale_python_mypy_args
+endfunction
+
+call g:ale#linter#Define('python', {
+\   'name': 'mypy',
+\   'executable': 'mypy',
+\   'command_callback': 'ale_linters#python#mypy#GetCommand',
+\   'callback': 'ale#handlers#HandleGCCFormat',
+\})