DISC: add accessor attributes to Index for consistency with Series
Broken off of #17117 for discussion, xref #8162, #17061, recent mailing list [thread](https://mail.python.org/pipermail/pandas-dev/2017-June/000608.html).

With a datetime-like column we can access year, hour, ... with `self.dt.foo`.  With a DatetimeIndex (PeriodIndex, ...) we access these attributes directly `self.foo` without the `.dt`.  I'd like to add a `.dt` property to the appropriate Index subclasses so that these attributes can be accessed symmetrically.  i.e. instead of:
```
if isinstance(obj, pd.Index):
    year = obj.year
elif isinstance(obj, pd.Series):
    year = obj.dt.year
```
we can just use `year = obj.dt.year` regardless.

The implementation is three lines in core.indexes.datetimelike.DatetimeIndexOpsMixin`:
```
    @property
    def dt(self):
        return self
```

Thoughts?