Add bgcolor argument

[jbednar]

As far as I can tell, the Pandas .plot() API does not have a way to change the background color without dropping down to matplotlib. hvPlot appears to inherit this the same limitation. We can of course do .opts(bgcolor='black'), but I've found that in many examples that's the only use of .opts I have to resort to, and without it I wouldn't need to explain .opts. Can we have a bgcolor= argument to cover this case?