Add _.call (for use when chaining)

When chaining it would be useful to be able to pass the whole object through a function, kind of like _.tap except the value is replaced with the return of the function.

Implementation is trivial and of course it doesn't make much sense when not chaining.

Anyone think this is a good idea?
btw while chaining it you wouldn't be passed the `obj` if you implement it this way as you haven't used mixin. You'll probably want

```js
function(fn, context) {
    return fn.call(context, this._wrapped);
}
```