Add support for string interpolation.

[mattdeboard]

Currently, interpolated variables do not have any special syntax highlighting. This is bad.

@lunaryorn has an excellent guide here that could help immensely in getting this done.

[swsnr]

@mattdeboard Thanks, I'm flattered :)

Please feel free to steal from our Puppet Mode. It also has some other nice utilities for strings added by @bbatsov. Also, don't hesitate to ask me, via mail, Github, or Twitter.