plugin: new katex plugin for math rendering

[cmoog]

No description provided.

[oscarotero]

Hi, thanks for your contribution. A plugin for Katex is a great idea.
I use to store the new plugins to the https://github.com/lumeland/experimental-plugins repository, so they can be imported from there and tested before adding it to the main Lume repo. But in this case, I think it can be merged directly to the main repo.
I want to make some changes before merging but I will do in the comming days.