fire a an event called [transition-name]:end when a transition completes

Inspired by the following gist : https://gist.github.com/Rich-Harris/c5033b380e0c2e4e0749

This should allow users to do stuff like:

```
ractive.on('fly:end', function() {
  console.log('the fly transition has ended');
})
```

Instead of the more generic:

```
ractive = new Ractive({
  ...
  complete: function () {
    alert( 'All initial intro transitions have completed' );
  }
});
```