ENH/API: Decide what to return Period - Period subtraction
#### Code Sample, a copy-pastable example if possible

Split from #13071. Currently it returns int and loses freq info.

```
pd.Period('2011-03', freq='M') - pd.Period('2011-01', freq='M') 
#2
```
#### Expected Output

Current ideas are:
1. Return `Timedelta` adding support of `Monthly` freq.
2. Return `Perioddelta` newly created.
3. Return `DateOffset`.
#### output of `pd.show_versions()`

on current master.

CC: @jreback @MaximilianR 
