Add hooks for plugin authors

After this pull, users can add plugins to WCT via their config file like so:

```js
module.exports = {
  plugins: {
    "my-plugin": { ... }
  }
}
```

This is an improvement to #60. It turns out all coverage logic can be added to the npm plugin if WCT provides the right hooks.

This incorporates the suggestions @nevir provided in #60.

Checkout [web-component-tester-istanbul](https://www.npmjs.org/package/web-component-tester-istanbul) to see this in action.
Ah, sorry for the confusion:

`runSubSuite` is run within the iframe (so `window` is the iframe's window, `WCT` is the iframe's global, `parentWCT` is the index's copy of `WCT`). And that only executes on a successful load of the child `.html` file - so that's a good point to wire up any state

(remember - all tests also load `browser.js`, which is why we can get away w/ this)