Allow custom hyperlinks; Pass differential.diff-id into remarkup engine config

Summary: This allows extensions to have more options for generating custom
hyperlinks.

Test Plan:
custom-inline rules are moved before default rules. Test existing products which
implement custom rules.
Make sure you use "$this->getEngine()->storeText()" in rules.

Reviewers: epriestley, jungejason

Reviewed By: jungejason

CC: aran, epriestley, emiraga, jungejason

Differential Revision: 1024