Support math extension

[rhysd]

This PR adds support for mathematical expressions which was introduced to GitHub recently as Options::ENABLE_MATH extension.

This extension supports both inline-level and block-level expressions:

Inline-level:  $\sqrt{3x-1}+(1+x)^2$

Block-level:

$$\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)$$

They are rendered as follows on GitHub:

Inline-level: $\sqrt{3x-1}+(1+x)^2$

Block-level:

$$\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)$$