Add support for flake8 per-file-ignores

[brandonwillard]

This PR implements quick and dirty support for flake8's per-file-ignores option.

Changes to the list-type config parsing were necessary. Specifically, the parsing for multiline config settings, e.g.

[flake8]
ignores =
    E101,
    E102

needed to be altered to allow for multiple commas on a single line, because per-file-ignores takes a multiline list containing file names and comma separated lists of errors to be ignored:

[flake8]
per-file-ignores =
    foo.py: E101, E102
    bar.py: E101

[ccordoba12]

Thanks for you contribution @brandonwillard! Looks good to me, except for some minor comments.

[brandonwillard]

What is this test_debounce that's failing?

[andfoy]

That is a flaky test that is unrelated to this PR, I've just re ran the CIs

[andfoy]

Thanks @brandonwillard!

[reagle]

When might this be released? I'm looking forward to using it in my editor. 😄