Update fontURL, support for reconfiguration

[pkra]

Fixes #323

[pkra]

I ended up going with a simple replace in the config() method. Not sure if there are objections to that.

@dpvc is there neeed for config()to be synchronized with the queue in general?

[dpvc]

I don't think this change should be made. As mentioned in #323, just as with MathJax in the browser, configuration should be done before any typesetting occurs. There are many things that can't be reconfigured after typesetting begins; for example, you can't change the extensions after the initial typeset. I would not complicate the code (and add maintenance costs) for one feature while leaving all the others unaddressed. Is there really a use case for changing the font url between typesets? If so, you can always do mjAPI.start() again after the reconfiguration.

So I recommend not applying this PR.

[pkra]

See my comments on the issue.

[pkra]

Closing in favor of #326