rolling: allow control over padding

Code Sample, a copy-pastable example if possible

import numpy as np
import xarray as xr

x = np.arange(1, 366)
y = np.random.randn(365)
ds = xr.DataArray(y, dims=dict(dayofyear=x))

ds.rolling(center=True, dayofyear=31).mean()

Problem description

rolling cannot directly handle periodic boundary conditions (lon, dayofyear, ...), but could be very helpful to e.g. calculate climate indices. Also I cannot really think of an easy way to append the first elements to the end of the dataset and then calculate rolling.

Is there a way to do this? Should xarray support this feature?

This might also belong to SO...

Start with the rolling API shown in the issue and reproduce the centered dayofyear example to inspect the current boundary behavior. Define how configurable padding should handle periodic boundaries, then verify that rolling climate-index calculations produce the intended results for the example.