Separate storing of rendered hyperlinks in remarkup hyperlink rule

Summary: Benefit of this separation is that custom hyperlink rule may decide not
to process given link. Such text will continue to be accessible for other
remarkup rules (e.g. imageproxy, youtube, etc.)

Test Plan: Checked that links are still properly displayed in differential. This
change should not affect existing applications.

Reviewers: jungejason, epriestley

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: https://secure.phabricator.com/D1389