add placeholder support to _.partial

This is another implementation of #1276/#1318. @jashkenas requested a simpler solution[¹][1]; hopefully this fits the bill.

Possible example for the docs, should this pull request be accepted:

```javascript
var product = _.partial(_.reduce, _, function(a, b) { return a * b; }, 1);
product([2, 3, 7]);
=> 42
```


[1]: https://github.com/jashkenas/underscore/pull/1318#issuecomment-27896798