Add text linting to Arcanist.

This introduces a .arclint file used by Arcanist to run a handful of checks on patches submitted to Phabricator. To keep it portable and avoid annoying contributors, most of the linting will happen through Jenkins. This commit uses built-in text linting that checks trailing whitespace and end of files.

Reviewed By: leper

Differential Revision: https://code.wildfiregames.com/D6