Feature: .innerFields on static math instances

Static math instances now have a `.innerFields` list of the
`\MathQuillMathField{}`s that are inside.

Also add the optional name argument to `\MathQuillMathField[name]{contents}`
so that they are accessible by name on `.innerFields`.

See test cases.

(Also refactor initialization of the MathQuill API objects a little
in `publicapi.js`.)