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.
The `iframe`/`contentWindow` may not always be there (error cases). Let's create the reference when we boot up the subSuite (nee bundle).

Probably clearer to set up another hook on this class (`initialize`?) that we call from `run.js`'s [`runSubSuite`](https://github.com/Polymer/web-component-tester/blob/master/browser/mocha/run.js#L108-119) (which has the subSuite's `WCT` on hand too). We can associate `share` and any other info at that point.