Proposal: use `<span class="math">` instead of `$$`

[s-cerevisiae]

This should generally be faster and more stable than letting Katex look for delimiters in string. Which are not even paired.

However we need to take care of math strings in alt-texts so this should better be done after sorting out #45.

[kokic]

Yep. However, I will still leave the auto-render plugin in some way :)