Add page to experiment with parsing

Summary:
I added this to be able to test parsing things easily, but I thought it
might be useful to include it in the repo, like the one KaTeX has (which
I took a couple things from).

It outputs a couple useful pieces of parsed info:

* the AST
* the printed representation (print())
* the TeX (until it's removed)
* whether the expression is considered simplified
* the simplified form of the expression
* the JSON representing the parsed info (useful for seeing hints and
  other things)

Test Plan:
  * Run a local server and open /experimenter.html
  * See the info show up as you type an expression

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D6049