R1:b1d4d5c00ce1

Add an "{anchor #xyz}" rule to Remarkup

Summary: Ref T13410. Fixes T4280. Allows you to put a named anchor into a document explicitly.

Test Plan: Used `{anchor ...}` in Remarkup, used location bar to jump to anchors.

Maniphest Tasks: T13410, T4280

Differential Revision: https://secure.phabricator.com/D20825