ENH: Allow list of keys in Dataframe.xs for each level
```
arrays = [np.array(['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux']),
            np.array(['one', 'two', 'three', 'two', 'one', 'two', 'one', 'two'])]

df = pd.DataFrame(np.random.randn(8, 4), index=arrays)
```
It would probably be nice to allow for 

`df.xs([['bar', 'baz'], ['one', 'three']], level=[0,1])`