Add a value transform function to groupBy

Hey @dtao! I'm curious what you think of this tiny modification.
It allows for the following:

```javascript
var input = [
  { group: 'g1', value: 'Awesome stuff',    irrelevant: 'blargh' },
  { group: 'g1', value: 'Not to be missed', irrelevant: 'blargh' },
  { group: 'g2', value: 'Check it out',     irrelevant: 'blargh' },
  { group: 'g2', value: 'This is amazing',  irrelevant: 'blargh' }
]
lazy(input).groupBy('group', 'value').toObject()
// => { g1: [ 'Awesome stuff', 'Not to be missed' ],
//      g2: [ 'Check it out', 'This is amazing' ] }
```

Again, thanks for your awesome module!

Dunno if you were at ForwardJS here in SF a few weeks ago, but someone
gave a talk and mentioned lazy evaluation in javascript utility libraries.
I only caught the tail end of it, so I'm not sure what was said, but I'm
glad more people are thinking in this way now :)