Add syntax highlighting for the "Robot Framework" language

Summary: Adds syntax highlighting for the Robot Framework via pygmentize.

Test Plan:

> pygmentize -L | grep robot
* robotframework:
    RobotFramework (filenames *.txt, *.robot)

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11761