Support instance methods on @resolve

For computationally expensive operations, it'd be good to have:

``` js
@resolve("options", (props) => {
/* Something that does lots of processing */
}, {
shouldComponentUpdate: function(props) {
return this.props.choices.length !== props.choices.length;
}
})
class ChoiceWidget extends React.Component {
...
```

Otherwise, all of the work gets done & the main component has no chance to bypass the effort.