Item loaders new values argument

There is a bug in the current `ItemLoader` if you instantiate it with an `item` like `{'foo': 1}` and then call `add_value('foo', 2)` the return of `load_item()` will be something like: `{'foo': 2}` and we would expect something like: `{'foo': [1, 2]}`.  In order to address this we would add a new constructor argument called *values* which will allow loading data into the loader while preserving backwards compatibility with the *item* argument.