API: add 'level' kwarg for Index.isin method
xref #7886

It would be nice to be able to write:

``` python
s[s.index.isin(['a', 'b', 'c'], level='foo')]
```

instead of

``` python
s[s.index.get_level_values('foo').isin(['a', 'b', 'c'])]
```

The former variant is not only shorter but is also potentially more performant: one only has to look up levels in given container and then operate on labels only avoiding instantiation of sub-index completely.

Default value should be `None`, plain `Index` objects should only accept `None`, `0`, and, `Index.name` for the sake of consistency (the last one is especially arguable).
