Small change to parser behaviour

This changes how proxy events are represented in parsed templates, from

```js
Ractive.parse( '<div on-foo-bar="baz:qux"></div>' );
// -> [{"t":7,"e":"div","v":{"foo":{"n":"baz","a":"qux"},"bar":{"n":"baz","a":"qux"}}}]
```

to

```js
Ractive.parse( '<div on-foo-bar="baz:qux"></div>' );
// -> [{"t":7,"e":"div","v":{"foo-bar":{"n":"baz","a":"qux"}}}]
```

As well as being more efficient this works around a downstream bug with template optimisation.