Ale: Turn Flake8 PyCodeStyle errors into warnings (or add option)

[julianandrews]

flake8 combines a number of tools including the PyCodeStyle style checker. PyCodeStyle classifies some issues as warnings and others as errors, which makes sense in the context of a style checker, but in the context of linting the PyCodeStyle E### errors should really just be warnings since they're really not errors like those from pyflakes. I'd advocate just reporting all E### errors as warnings.

Alternatively, a config flag (maybe g:ale_python_flake8_codestyle_warnings?) could be used to convert the code style errors into warnings.

[w0rp]

See :help g:ale_type_map for changing the type of problems for any linter.

[julianandrews]

Ohh, awesome, thanks!