Mathjax improvements

Added some configuration options to the mathjax plugin.   They are documented in the plugin-use.md documentation. 

The defaults are as they used to be, but personally I think it would be cleaner if 
it least the default of mathjax-config is changed to NIL.

Also, feel free to suggest better names for the options.  Or if you have any other feedback.

Kind regards,
Wim Oudshoorn.