Add Official Support for `$...$` for Inline Math and `$$...$$` for Display Math

[RoyiAvital]

As a continuation to #413.

It would be great to have ability to use the same notations as used in GitHub for math:

1. $...$ - In Line Math.
2. $$...$ - Display Math.

This will also make the notations more aligned with LaTeX and other MarkDown parsers.