Implement Result#pluck

This little method is only for convenience, but I find myself writing this enough by hand that I thought I would make the proposal.

Behavior basically matches the [Underscore.js pluck method](http://underscorejs.org/#pluck). I'm not sure what other libraries use the term "pluck," but hopefully the meaning is intuitive to others besides myself.

I can add any needed documentation to the PR before it is merged. I just wanted to find out first if this change would have a chance of being accepted. All suggestions welcome.