Support third party reporters

Support third party reporters by wrapping them in a closure function to expose the 3 CommonJS/A module variables - `exports`, `module`, and `require` with very minimal objects. `require` will only try to lookup other reporters. `process` is also in the closure from `Mocha.process`.

I also cleaned up the documentation a bit - basically all builtin reporters will mostly "just work" now so no need to focus so much documentation on that. We were missing some that work anyways (`markdown`, `landing`, and `progress`).

Fixes #81