html backend: persistent katex macros

[dlesbre]

Previously, if you defined a macro using \gdef in a math block, that macro would only be scoped to that block in the HTML KaTeX render. This PR adds persistent macros, allowing you to reuse that same macro in all subsequent math blocks.

Locally scoped macros are still possible using the \def command.

This is in accordance to the KaTeX documentation on macros:

Macros defined by \gdef, \xdef, \global\def, \global\edef, \global\let, and \global\futurelet will persist between math expressions.

The solution I used here is explained in the KateX API documentation.