math formula support

[docsAsCode]

add support for mathematica formula.

in markdown :

$$
a = b ^ 2 + c
$$

In RST :

.. math::

   a = b ^ 2 + c

[docsAsCode]

• asciidoctor document must have stem attribute
• asciidoctor-pdf must use -r asciidoctor-mathematical
• pandoc must convert in asciidoctor (not in asciidoc)