Add --everything support to 'arc lint'

Summary:
When adding arcanist support to a new project or adding a new linter,
it's helpful to be able to run new linters against the entire codebase. This
patch adds support for this with an '--everything' option, similar to 'arc unit
--everything'

Test Plan:
Run 'arc lint --everything' and check out the code. Optionally dump
the paths to test in the current lint engine's buildLinters() function to
demonstrate that it's receiving all files in the project rather than just the
changed and/or specified ones

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D6592