Support macros when prerendering

[rnburn]

Macros aren't passed to the katex command line if katex_prerender=True.

They could be written to a file and passed using the --macro-file CLI option.

Example macro file

\abc: 123
\xyz: \mathbf{XYZ}

[hagenw]

Thanks for reporting. Seems like pre-rendering from Python code is in general far from optimal, compare #37.

There should be a solution to the problem reported here. We have to look for macros: in the katex_options string to extract them, then write them to a temporary file and if this file exists add the --macro-file option. Not very nice, but I guess still better then not including the macros.