Add spell check linter

Summary:
Inspired by http://news.ycombinator.com/item?id=3464671 and a lot of
diffs I've seen @ FB, I've added a spell checking linter. To reduce
false positives, it's only a blacklist. Still, it catches a large
number of 'issues'.

Test Plan:
Unit tests. Ran on FB's codebase. No false positives
noticed but a lot of cases caught.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley, jack

Differential Revision: https://secure.phabricator.com/D1409