Add transform for ES6 templates

Summary:
See http://people.mozilla.org/~jorendorff/es6-draft.html#sec-12.1.9
https://github.com/dherman/tc39-codex-wiki/blob/master/data/es6/template-strings/index.md
http://wiki.ecmascript.org/doku.php?id=harmony:quasis

I really only want this for multiline strings.