Allow users to pass backend option to .plot()

[jsignell]

Once you have installed another plotting library with a pandas.plotting entrypoint it would be nice to be able to toggle back and forth between different backends even within on session.

What currently exists

pd.options.plotting.backend = 'holoviews'

Proposed additional API

df.plot(backend='holoviews')

This would be particularly useful when a particular backend doesn't support a certain type of plot, or you need to use a backend to get some specific functionality but don't want to use if for your whole notebook.

@datapythonista - @TomAugspurger mentioned that you might have opinions on this.

[datapythonista]

I'm ok with this, sounds reasonable.