Close #8: math rendering

[bonartm]

Closes #8. Currently, using this branch, one can include latex within dollar signs (single $ for inline, double $$ for block):

$x=2$

• Ideally, I'd like to include it without using backticks.
• By including katex, the bundle size doubles to half a megabyte. I'd like to dynamically load the external libraries (syntax highlighting, katex) based on the users' choice. So users who don't need the features benefit from a small library.

[bonartm]

With the new marked.js extensions markedjs/marked#2043 it became much easier to customize the library. Now formulas can be written without backticks.

[bonartm]

Implemented a multi output bundle that allows the users to register extensions (syntax highlighting + math formulas) based on their needs. This keeps the file size of the main library small. Output format is now esm and documentation is updated accordingly.