Don't run disabled lint rules

Summary:
We always generate all messages and then filter them out based on minimum severity.
It's lots of useless work, especially in commit hook mode where we are interested only in errors.

Test Plan:
  $ arc lint --cache 0 --severity error ArcanistXHPASTLinter.php
  0.406 s before, 0.074 after

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4963
Don't run disabled lint rules in other linters.

Summary: D4963 for other linters.

Test Plan: Saw time 0.001 instead of 0.113 in spellcheck linter.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4965
Merge branch 'master' of http://github.com/facebook/arcanist