Conditional proxy events (#618)

Closes #618. This PR has support for addEventListener/removeEventListener if there is no "action" specified:

```html
<div on-click='{{#condition}}foo{{/}}'></div>
```
For custom events, it will call the custom event fn and resulting teardown as it would add/remove.

@Rich-Harris Let me know if I misused any of the idomatic ractive virtual dom methods.