ENH: Plotting backend options

[jsignell]

Since pandas has implemented entry_points based plotting backends, it seems reasonable that xarray would do the same. This would make it even easier to produce holoviews plots (rendered in bokeh via hvplot), by using the plot method rather than by importing hvplot directly.

Example

import xarray as xr
air = xr.tutorial.open_dataset('air_temperature').load().air
xr.options.plotting.backend = 'holoviews'

air.isel(time=500).plot()