Better default formatting of timedelta with plot method

[ahuang11]

Currently, it shows nanoseconds.

import xarray as xr
ds = xr.tutorial.open_dataset('air_temperature')
ds.coords['tau'] = ds['time'] - ds['time'][0]
ds.mean(['lat', 'lon'])['air'].swap_dims({'time': 'tau'}).plot()

[dcherian]

dupe of #2898. I think this should be fixed by matplotlib though we could copy whatever pandas is doing in the interim.