Add katex support

[stakaz]

It would be great to have katex support. It is way faster in rendering math code when dealing with physical texts. I am aware of markdown-preview-enhanced but there are much more things which disturb my work flow than with markdown-preview-plus. The most important thing is live preview with markdown.

When I pass --katex to pandoc it works very well even now. However, the display-math is not centered and put on a new line. I think this could be fixed very fast in just editing the css for the preview but I am not sure about it. This would be the fastest "workaround" and the first step towards katex.

Thanks for the great package!

[lierdakil]

While I'm pretty sure integrating KaTeX would be rather straightforward (moreso than MathJax), I'm afraid this project has been in low-power maintenance mode for a while due to all maintainers being busy with other projects and/or life in general.

So PRs are welcome and encouraged, since waiting until people actually responsible find some time to work on this can easily take months if not years.

So yeah, help wanted, and I'm pretty sure it should be rather straightforward, just add an option to use KaTeX and plug it in everywhere instead of MathJax. Note: Pandoc may require additional attention, since currently it's rather hard-wired to be called with --mathjax.

[alvis]

How about merging https://github.com/abejfehr/markdown-preview-katex? This is a fork of MPP and seems like promising, though it is now depreciated.

@abejfehr: Would you mind sharing your awesome work by submitting a PR?