EA interface: prepare for unique(..., return_inverse=True)
While adding the keyword and functionality (beyond the cython backend) is only gonna happen past v.0.24, I think the EA interface should be adapted to allow for the possibility of the `return_inverse`-kwarg.

The default implementation could just recommend to start with
``` 
if return_inverse:
    raise NotImplementedError('this array type does not yet support `return_inverse=True`')
```

Of course, this could be added along with the other changes to `.unique`, but I was wondering if this should already be prepared before v.0.24 so that EA authors (who will probably expect the EA interface to be more stable with v.0.24) can already prepare for it.

@jreback @TomAugspurger 