Allow pluggable manager for TaggableManager.

Add a `manager` kwarg to `TaggableManager` to allow easy subclassing of `_TaggableManager`.

I ran into a situation where I wanted to subclass `_TaggableManager` to add some magic to the `add`, `remove`, `set` methods of the manager. But the only way to do this was to subclass both `TaggableManager` and `_TaggableManager` and override `TaggableManager.__get__` to point to my new subclass.

This small change defaults to the normal `_TaggableManager`, but lets you pass along a manager of your own, allowing me to do this:

`tags = TaggableManager(manager=_SprintlyTaggableManager)`

Thanks!