revisit passing data along to callback

[pkra]

While #239 was resolved, the original goal (to pass potentially unrelated data along) was not achieved.

The relevant code is https://github.com/mathjax/MathJax-node/blob/master/lib/main.js#L832-L835. The main problem seems to be that the input object is not actually passed to the callback but rather an intermediary object is built, based on the defaults (with values from the input data).