Add aural "+" and "-" hints to unified diffs for users who use screenreaders

Summary: See PHI160 for discussion.

Test Plan:
With ?__aural__=1, saw aural hints:

{F5229986}

Without, saw normal visual diff.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18718