support `\[` and `\]` block math and `\(` and `\(` inline math

[ag2s20150909]

support \[ and \] block math and \( and \( inline math

https://gohugo.io/content-management/mathematics/#step-4

This is an inline \(a^*=x-b^*\) equation.

These are block equations:

\[a^*=x-b^*\]

\[ a^*=x-b^* \]

\[
a^*=x-b^*
\]

These are also block equations:

$$a^*=x-b^*$$

$$ a^*=x-b^* $$

$$
a^*=x-b^*
$$