* Add boolean option "extracomma" that warns on trailing comma, even in ...

...ES5 mode.  This is for developers who want to avoid trailing commas for style reasons, even though they're allowed in ES5 code.

* Also add unit test in tests/stable/unit/options.js with fixture file extracomma.js

This addresses https://github.com/jshint/jshint/issues/1131. I've run unit tests and lint on this code.