Add option to specify font to load for SVG and CHTML output.

[dpvc]

This PR converts the font option to the output jax to specify the name of the font component to load (with a default of tex). The old font option was the FontData object or instance to use, and this PR changes that to the fontData option instead. Backward-compatibility is maintained so that code that uses the older font option will still work. Because font now specifies a component, it can't be used in node applications that load the MathJax modules directly (you must use the original mechanism of passing the FontData object via fontData, as those applications did using font in the past).