Highlight monospace first

Summary:
It should be possible to write D1234 and others without making links from them.
Depends on D1913.

Test Plan:

`D1234`
`<a href="">`

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1915