Add ArcanistLintSeverity::SEVERITY_AUTOFIX.

Summary:
Xcode (a popular code editor on Mac OS X) has no facility
to trim trailing whitespace automatically.

This adds a new lint severity "AUTOFIX" that's between
WARNING and ERROR. When running the linter, any lint message
whose severity is AUTOFIX will automatically be patched.

Furthermore, if all lint messages returned from the engine are
AUTOFIX, we'll automatically amend HEAD with the patch.

Test Plan:
arc lint on files with and without trailing whitespace,
with and without UTF-8 contents to confirm those still error

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2125