Split Remarkup rule for document link from hyperlink

Summary:
We have other rules dealing with hyperlinks (PhabricatorRemarkupRuleProxyImage) which collides with one rule for hyperlink:

• If we apply proxy image first then it creates [[ storedText ]] which builds an invalid link.
• If we apply hyperlink first then there's nothing left for proxy image.

Test Plan: [[ http://example.com/a.png | a ]]

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3191