Support for lint messages which are always shown

Summary: If one wishes to implement a linter which finds unused resources or variables the current scheme does not work as the lint engine filters all changes from lines which were not introduced in a diff. To solve that, I've added an "always show" configuration for a lint message allowing creation of such linters.

Test Plan:
1. Created a custom linter for finding unused Android resources in a project
2. Ran arc lint with linter added and received warnings as expected

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6119