ENH: .interval accessor
similar to how we work with ``.dt``, ``.str``, and ``.cat`` accessors, it might be nice to expose an ``.interval`` accessor; in particular this might make nice indexing expressions, xref to #16316 

http://stackoverflow.com/questions/44088460/interval-datatype-in-pandas-find-midpoint-left-center-etc/44088970#44088970

```
In [13]: df = pd.DataFrame({'month': [1, 1, 2, 2], 'distances': range(4), 'value': range(4)})

In [14]: df
Out[14]: 
   distances  month  value
0          0      1      0
1          1      1      1
2          2      2      2
3          3      2      3

In [15]: result = df.groupby(['month', pd.cut(df.distances, 2)]).value.mean()

In [16]: result
Out[16]: 
month  distances    
1      (-0.003, 1.5]    0.5
2      (1.5, 3.0]       2.5
Name: value, dtype: float64

In [17]: pd.IntervalIndex(result.index.get_level_values('distances')).left
Out[17]: Float64Index([-0.003, 1.5], dtype='float64')

In [18]: pd.IntervalIndex(result.index.get_level_values('distances')).right
Out[18]: Float64Index([1.5, 3.0], dtype='float64')

In [19]: pd.IntervalIndex(result.index.get_level_values('distances')).mid
Out[19]: Float64Index([0.7485, 2.25], dtype='float64')
```