Adding derived datatime for xarray

[jsignell]

Closes #54

Allows using derived datetime for xarray objects

import xarray as xr
import hvplot.xarray
ds = xr.tutorial.open_dataset('air_temperature')
plot = ds.hvplot.violin(y='air', by='time.month')
plot

ds.mean(dim=['lat', 'lon']).hvplot(by='time.hour')

data.hvplot.scatter(y='air', x='time.dayofyear', by='time.year')

[philippjfr]

This looks great!