Implement quarterly frequencies for cftime

[tlogan2000]

Currently cftime offsets have no options for quarterly frequencies (QS-DEC ... etc) as in xarray/pandas
see : http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases

@jwenfai has recently worked extensively to implement xarray/cftime resampling (#2593) but this will remain limited as quarterly frequencies are currently not supported under xarray/coding/cftime_offsets.py

[spencerkclark]

Yes, these would be a welcome addition 👍.