Add pandoc settings.

[Robin-des-Bois]

It would be nice if it would be possible to invoke pandoc with custom extensions and settings configurable over the web front end. In the beginning maybe just a sanitized string that gets passed to pandoc.

Use case: I would like to be able use latex-style macros outside of mathmode and I think there is an extension for that...

[yoavram]

I’ll be happy to accept a PR