API: add 'level' kwarg to 'Index.isin' method

closes #7890

This PR adds `level` kwarg for `Index` objects as discussed (briefly in #7890):

```python

Index.isin(iterable, level=None)
```

### Summary
- for non-`MultiIndex` classes:
  - no functional changes in the actual membership test
  - valid `level` values are `None`, `0`, `-1` and `self.name`
  - if `level` doesn't match the name, it's a `KeyError` (was `AssertionError` before)
- for `MultiIndex` classes:
  - if `level=None`, elements of `self.values` (tuples) are used
  - otherwise elements of corresponding level are used
  - valid `level` values are the same as in `MultiIndex.get_level_values` method, `-self.nlevels..(self.nlevels - 1)` plus all unique index names

### TODO
- [ ] add to documentation
- [x] optimize MultiIndex case
yep, but 'notes' like this don't work in the 'Parameter' section (it is then interpreted as another parameter, the format is quit strict), therefore your have the 'Notes' section, as you did now :-)