Include default font configuration so that it will be loaded when input/chtml or input/svg is loaded

[dpvc]

This PR adds the default font to the default configuration for node-main so that when an output jax is loaded, it will load the default font automatically (rather than the "no-font" that is built into the input/chtml and input/svg components). This allows you to do

require("mathjax-full").init({loader: {load: ['input/tex', 'output/svg']}}).then(...)

without having to include configuration for the font by hand. You can still override the font by setting the loader.paths and output.font values in the argument to init() if you want to use a different font.

[zorkow]

lgtm.