Add a simple FILTERS pragma example.

This includes examples of both section and interpolation filters.
Require HHVM to pass now.

https://github.com/facebook/hhvm/pull/1860
Increase test coverage.

… fix a bug exposed by increased test coverage :-/
Bump to v2.7.0
Merge branch 'release/2.7.0'
Merge remote-tracking branch 'upstream/master' into original

Conflicts:
	test/Mustache/Test/EngineTest.php