Fix UTF8 file detection

Mostly generated from @yongkangchen's test file, I tracked down fixes in `ag` from about a year ago for this: https://github.com/ggreer/the_silver_searcher/pull/267

Closes https://github.com/gjtorikian/isBinaryFile/issues/16.

@benogle you'll probably want to pull this in to the various Atom libs using it (three?). I'm sending it out as `2.0.2`.