lib/markdown2: introduce LaTeX/Maths mode

[Morriar]

This PR is a follow up of #2720 and introduces a LaTeX/Maths mode for lib/markdown2.
The goal is to handle #791 when it will be merged with the documentation tools.

The maths mode, when enabled, uses the tex2im tool to render LaTeX equations as images and includes them into the HTML output.

So one can write:

This is a LaTeX equation: $\\psi_{tot}(x,-t_0,r) = \\frac{1}{(2\\pi)^2}$.

And the output will be:

<p>This is a LaTeX equation: <img alt="\\psi_{tot}(x,-t_0,r) = \\frac{1}{(2\\pi)^2}" src="b123b75db5eb96ac4496d017a18633b5.png" /> baz</p>

Where the image will be:

To work this PR requires tex2im to be installed and added to the path.