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