Add YAML support

[wmertens]

TL;DR: With YAML you can write begin: \bstart\b instead of begin: "\\bstart\\b".

I know YAML and CSON are quite similar, but YAML allows strings to remain unquoted (http://www.yaml.org/spec/1.2/spec.html#id2788859), which allows writing bare regexes and makes writing things like https://github.com/wmertens/sublime-nix/blob/master/nix.YAML-tmLanguage a lot easier.

It is also what is used by many grammars for Sublime, via the AAAPackageDev tools, and github allows it in linguist.

Here's a YAML-tmLanguage highlighter used on github to highlight a YAML-tmLanguage file. Ignore the regex illegal character issues, the highlighter isn't 100% right since it does parse cleanly, as evidenced by http://nodeca.github.io/js-yaml/ which is what is linked from the yaml.org website.