Support for Lambdas

Hey,

I'm not able to edit the lambdas here. I want to be able to do something like:

```js
var squared = _.map(input, (x) => x * x); // this is my ultimate goal
var squared = _.map(input, lambda(x) x * x); // this is what I am trying to achieve right now
```

Can you suggest how to go about doing this?