Allow renderering Remarkup to text

Summary:
This will be useful mainly for `D123` and `{D123}` in text e-mails.
I've consider also other approaches:

# Convert the rendered HTML to text - we lose some information when converting Remarkup to HTML, e.g. whether `##` or ##`## was used.
# Return the Remarkup intact, e.g. in simple table or lists (after applying inline rules) - tables and lists formatted by computer usually look better than the ones from users.
# Create a separate text renderer - HTML and text renderers share lots of regular expressions and code so splitting them would create more complexity.

The mode is a bitmask to allow something like `self::MODE_TITLE | self::MODE_LINK` in future.

Test Plan: Old and new unit tests.

Reviewers: epriestley, edward

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2617

Differential Revision: https://secure.phabricator.com/D5365