Add "jump to table of content" keyboard shortcut

Summary:
Many times when I'm reading a big diff, I want to go to the
TOC. Add it.

Test Plan:
can navigate with 't'. It also shows up in '?'

Revert Plan:

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: nh, aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2335