As discussed with @Andrewwango in PR #611, the functions plot and plot_inset now share many overlapping features and parameters
We see two solutions:

1. merge into one function plot with optional parameters for the insets
2. lump all the parameters of plot into a **plot_kwargs in the declaration of plot_inset

I think both approaches have trade-offs:

1. breaks backward compatibility, meaning all current uses of plot_inset have to be fixed manually
2. preserves backward compatibility, but makes the API less transparent. Users need to consult plot's docstring to know what parameters they can use in plot_inset