Add fontExtensions configuration option. (mathjax/MathJax#3410)

[dpvc]

This PR adds a fontExtensions configuration that makes it easier to specify a font extension to load (e.g., mathjax-euler). It sets up the extraLoads package data to make sure the extension loads after the font itself. The current method of including the extension in the loader.load array doesn't work properly as it can be loaded before the font arrives. That can be handled by setting up dependencies and paths in the loader configuration, but it is harder than it should be, so this makes it easy.

Resolves issue mathjax/MathJax#3410.

The original section that was used to configure a font has been moved to a configFont() function, and a new configExtensions() function uses it to set up the font extensions.