Add `_.append` and `_.prepend`

As there is _.pull and _.remove to manipulate arrays, I thought it might be good idea to also offer functions to add new elements.

Use case with AngularJS/SocketIO
```
socket.on('list:init', _.partial(_.append, $scope.list)); // Multiple elements
socket.on('list:new', _.partial(_.append, $scope.list)); // Single element
```