when run in browser, how will Mocha resolve plugins?

Currently you're expected to get a function from any given (non-bundled) plugin, and pass that into `mocha.use()`.  But if we're bundling plugins for the browser, they can attach themselves to a global namespace (ostensibly `window.mocha`).  I think this is reasonable.  

I'm trying to implement but it's currently broken; see `wip` branch
