Support instance methods on @resolve

[ericclemmons]

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

@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.