Implement text mode for Phabricator Remarkup rules

Test Plan:

D1
{D1}
{C1}
{F1}
[[ Test ]]
iiam
{meme, src=iiam, above="I\'m not always", below="But I am"}
@{function:pht}
@vrana

Reviewers: epriestley, edward

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2617

Differential Revision: https://secure.phabricator.com/D5392