math: support output to markdown

[melisgl]

Also, parameterize the output markers.

[melisgl]

I added support for $x$ and $`x`$ syntax (rendered here as $x$ and $x$). The previous inline syntax $$x$$ (rendered here as $$x$$) also works as inline math within a paragraph and display math as a block element. I also added tests.

[melisgl]

Hold on. Escaping is missing.

[melisgl]

In Pandoc's tex_math_dollars extension, the dollar sign can be escaped with a backslash:

\$x_0\$

Same thing in GitHub Flavored Markdown (GFM) renders as $x_0$. However, adding a zero-width space just before the first backslash escape as in ​\$x_0\$ (obviously, the zero-width space is not readily visible) works for both Pandoc and GitHub: ​$x_0$.

Thus, the solution I'm proposing is the support backslash escape $ signs when parsing and emit backslash escaped dollar signs when needed plus the zero-width space for GFM.

[melisgl]

I pushed a new version, that parses escapes properly. However, printing seems to have no provision for extended escape chars or am I missing something?

[melisgl]

This latest version does proper escaping in printing too, but it does not yet have the zero-width space workaround for the lack of escaping $ on GitHub. Shall I include that in the change? I can add this workaround in PAX, too.

[3b]

not sure about the github escaping thing, maybe with an option to disable it?

[melisgl]

The zero-width space implementation wouldn't fit nicely into the existing escaping logic, and it needs the output stream to be unicode capable . I think I'll implement this in PAX after all.