R1:acde6659052a

Add Source Sans Pro Italic Fonts

Summary: Improves Source Sans Pro italics rendering

Test Plan: Tested a Phriction document with normal and bold italics.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11572