Rename .controller on API objects to .__controller

This isn't part of the public API, it's only exposed because "we're all
consenting adults here", but it should be namespaced differently from
the public methods, especially seeing as our own README example suggests
adding extra properties to MathQuill API objects:
https://github.com/mathquill/mathquill/commit/6011db1ee9568f37e13eb3f2c6829f6f04939933#diff-04c6e90faac2675aa89e2176d2eec7d8R182

So, namespace by prefixing with two underscores, Python-style, like we
did with `.__options`:
https://github.com/mathquill/mathquill/commit/c60fa6cbe4c43a3ed8ef5bfdcee1cfb1d953648f#diff-9e90820baf0fc358322430e6d2b3beefR51