Feature: add plugins like katex

I would like tu use Katex with ngx-markdown the same way I can use it on StackEdit.io. Is it planned to have some docs about how to add plugins or how to add new key (in StackEdit, Katex syntax is between $$) to execute some logics ?

## KaTeX

You can render LaTeX mathematical expressions using [KaTeX](https://khan.github.io/KaTeX/):

The *Gamma function* satisfying $\Gamma(n) = (n-1)!\quad\forall n\in\mathbb N$ is via the Euler integral

$$
\Gamma(z) = \int_0^\infty t^{z-1}e^{-t}dt\,.
$$

I can try to do something and do a PR if you want but I would need some indications.

[jfcere]

Katex support is now live, please refer to issue #181 for details.

[avelow]

I will share my code when it will be done ! I will try to do something like what is done here https://github.com/linxiaowu66/marked-kaTex/commit/3f9d61a025ccc338ae45716f5af11047cd42c4e8#diff-81ab0a5aa39b7a91951fc28d2d151496

I would have prefer if it was possible to had a new type of block (like quote, code or whatever) but it should be easier to just override the code renderer function