Add an "eval" rule to Remarkup

Summary:
Ref T13658. This adds a simple expression evaluator to Remarkup and supports platform name expressions. The syntax is:

${{{strings.platform.server.name}}}

Note that this won't work inside code blocks (or literal blocks, or other block-level literal elements) right now, although it could be made to selectively (the ".path" expressions might be useful in documentation codeblocks).

Test Plan:

Reviewers: cspeckmim

Reviewed By: cspeckmim

Maniphest Tasks: T13658

Differential Revision: https://secure.phabricator.com/D21713