Add basic support for ECMAScript 6 Template Strings

ES6 is adding support for "Template Strings", where expressions are embedded directly inside a string.  This prevents the eyesore of endless string concatenation.

Here's the spec: http://tc39wiki.calculist.org/es6/template-strings/

Here's what this constructed URL for the Flickr API looks like in vim-javascript currently:

![screenshot from 2014-02-19 20 18 10](https://f.cloud.github.com/assets/364615/2214230/3a7cf0d4-99cd-11e3-8146-74a296fc2de8.png)

This PR adds very basic support for highlighting template strings.  It makes `asdf` look like a string, and identifies ${foo} within a template string.  Here's a screenshot after this PR:

![screenshot from 2014-02-19 20 17 50](https://f.cloud.github.com/assets/364615/2214231/3a877bbc-99cd-11e3-9bdb-0563bb174e5e.png)

I don't know what your plans are for incorporating ES6 syntax changes.  Feature branch?  New repo?  Let me know, I'm happy to contribute wherever!