Implement text mode for Phabricator Remarkup rules

Test Plan:
  lang=remarkup
  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