Expose PEP8, Flake8 and CSSLint engines to .arclint

Summary:
Ref T3186. Ref T2039. Allow these linters to be selected with `.arclint`.

Also allow severities to be set.

Also fix some other minor bugs.

Test Plan:
https://github.com/epriestley/arclint-examples
https://github.com/epriestley/arclint-examples/blob/master/.arclint

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T2039, T3186

Differential Revision: https://secure.phabricator.com/D6803