Feature request: add @keyup and @keyupScoped decorators

In my use case, I’d like to be able to toggle state based on if a modifier key is pressed, so it would be great to be able to do something like:

```
class MyComponent extends React.Component {

@keydown('alt')
setModeCopy(event) {
this.setState({ mode: 'copy' });
}

@keyup('alt')
setModeMove(event) {
this.setState({ mode: 'move' });
}
}
```

In actually use, I think I would use @keydownScoped and @keyupScoped.