[Feature request] Return figure and axes objects for each plot function

[firai]

Certain plot customizations are much more difficult or practically impossible without access to the figure (fig) and axis (ax) objects. Using matplotlib's gcf() and gca() is possible, but relying on these limits how opsvis output can be used within a larger context.

Can the fig and ax objects be returned with each of the plot functions? I understand that there may be a major rewrite that's about to be pushed; otherwise I would create a PR to return these objects.