Adding pro tips to Differential commit message

Summary: Randomly displaying tips about different features of arc.

Test Plan: Will test by running arc diff and seeing if the tips appear in text-based UI of arc

Reviewers: epriestley, AnhNhan

Reviewed By: AnhNhan

CC: aran, Korvin, AnhNhan

Differential Revision: https://secure.phabricator.com/D5351