ENH: Arithmetic operations on intervals
#### Is your feature request related to a problem?

I would like to be able to do arithmetic operations on intervals:
- `a + b`:
  `pd.Interval(3, 6) + pd.Interval(5, 7) == pd.Interval(3, 7)`
- `a - b`:
  `pd.Interval(3, 6) - pd.Interval(5, 7) == pd.Interval(3, 5)`
- `[a, b].union() / np.sum([a, b])`:
  ```python
  intervals = pd.arrays.IntervalArray.from_tuples([(0, 1), (1, 3), (2, 4), (5, 7)])
  intervals.sum() == pd.arrays.IntervalArray.from_tuples([(0, 4), (5, 7)])
  ```
#### API breaking implications

None AFAIK, since those methods are not implemented yet
