Bug 1026334 - Emit confirm/cancel events from gaia-confirm

I don't really like the idea of requiring content authors to pass in a className for this hook. Just an idea, what if we say that authors should add a ```confirm``` attribute to the button that would fire confirm actions, and every other button would fire a cancel event?

```
if (e.target.hasAttribute('confirm')) {
  // Fire confirm event
} else {
  // Fire cancel event
}
```