With this a \end{XXX} tag only matches a \begin{...} tag with the same
environment name. Effectively,

    \begin{equation*}
      I = \begin{pmatrix}
	1 & 0\\
	0 & 1
      \end{pmatrix}
    \end{equation*}

won't result in borked syntax.

* Switched to the new extension system
* Don't use <script type='text/math'>...</script> for math displays,
  since it's not recognized by MathJax 3.
* Add a 'uses_math' option that gets set if any math is used.

As a result all this extension does is stop math like text from being
processed by markdown. No more text replacements, since that's handled
by MathJax.

The AsciiMath delimiter is `..` which conflicts with MarkDown's code
delimiters.