Support Inline Equation and Equation Block

It seems more and more people are combining markdown and mathjax now. This modification is adapted from the original code fence parsing rules. It gives better highlighting in vim. 

Before 

![b](https://f.cloud.github.com/assets/1227160/491279/cc095646-ba0d-11e2-94d8-2b6d1af3a9a8.png)

After

![a](https://f.cloud.github.com/assets/1227160/491280/d5adfb8e-ba0d-11e2-9f09-2ffba63c6547.png)

Sine there are a lot `_` (underscore) in math formula, it is essential to isolate them in order not to other texts.