Fix and test public API method .cmd(...), at all

- `ReferenceError: latex is not defined` should be `cmd`, not `latex`:

https://github.com/mathquill/mathquill/commit/f4ce7e9647cb2d3d05ff47e0551a7ed7ee258057#commitcomment-5855896

- `.write()` on `Node`s takes in the cursor as the first argument, for a
  total of 3 arguments:

https://github.com/mathquill/mathquill/commit/ed69739573d5397000905c942a824c5d13602070#commitcomment-5855828
Organize 'public API' section of latex.test.js #test #indent

Recommend -w to view diff.
Fix and test .write() empty LaTeX like '  {{} }'
Re-fix #152, and test this time
Fix and test public API method .html(), at all
Merge branch 'dev' into fix.publicapi
Merge branch 'fix.publicapi' into dev