Support highlighting block strings

This patch fixes #159 issue.

Original highlighting

![block-strings-orig](https://f.cloud.github.com/assets/554281/1689351/b262d134-5e1f-11e3-89d9-eaf793156849.png)


Fixed version highlighting

![block-strings](https://f.cloud.github.com/assets/554281/1673107/de83cfa6-5ce0-11e3-803b-0dd452545937.png)