MathJax support

[Shouheng88]

It seems that this library doesn't support mathjax expression like $$\left(\frac{x}{y}\right)$$. One way to solve this problem is to replace '{' with '\{', but I don't think this is the best answer. So, is there any good way to solve this problem when processs mathjax parsing?

Example:
For expression $$A = \int_{a}^{b} \vec F \cdot d \vec r$$,
current result is: <p><span class="math" a="" b="">$$A = \int_^ \vec F \cdot d \vec r$$</span></p> expected result is <p><span class="math">$$A = \int_{a}^{b} \vec F \cdot d \vec r$$</span></p>.