Fix excessively aggressive capturing of hyperlinks in Remarkup

Summary: Fixes T4319. See that task for examples. The "ungreedy" hyperlink rule is still a bit too greedy.

Test Plan: Added failing tests, made them pass.

Reviewers: btrahan, Firehed

Reviewed By: Firehed

CC: aran

Maniphest Tasks: T4319

Differential Revision: https://secure.phabricator.com/D7974