Centralize highlighter fallback

Summary:
I think that this API is better because all highlighters don't need to provide a fallback to the default highlighter if anything goes wrong.

But the real motivation comes in the next diff.

Test Plan:

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var a = 1;

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2226