implement string interpolation highlight

Related issue #6 #97 #98.
Please see this patch.

### Original
![coffee-mode-before](https://f.cloud.github.com/assets/554281/356940/feab6b18-a11a-11e2-98e6-41dfc26244f0.png)


### Applied this patch
![coffee-mode-after](https://f.cloud.github.com/assets/554281/356942/0b48a020-a11b-11e2-9861-92bf2755de14.png)