katex: enable persistent macros

[qnxor]

See https://katex.org/docs/api.html#persistent-macros

Katex allows using \gdef, \def, \newcommand and the likes. However, if some \gdef\root{\sqrt} is defined in some math expression then it won't be recognized in subsequent math expressions unless katex.render() is called as in this PR (in my view this should be default behavior in Katex, as it's expected standard Latex behavior, but alas).

Basically, this PR allows one to define macros at the top of a post, then reuse them as many times as they wish afterwards (should work for subsequent posts too, though I haven't tested that) e.g.:

$\gdef\mylim{\lim\limits_{n\to\infty}}$

Text...

$$
\mylim \ln(1+x^n)=0
$$

More text reusing the macro inline $\mylim c_n = 1$.

And so on...

[pmusaraj]

Looks good, thanks @qnxor!