Two wishes: add option and border option

[ugroempi]

Hi Lawrence,

thanks for providing Homeric! In using the package, I encountered two wishes:

• it would be nice if one could add a doughnut plot to an existing plot (option add with default FALSE). That's straightforward to do: you simply make the plot call conditional on add=TRUE.

• it would be helpful to be able to have borders arround the slices. While I normally prefer the plot without borders, I have an application for which red and green have to sit next to each other for external reasons, and it would be very colorblind-friendly to be able to separate them by a border. Maybe an option slice.border.col that defaults to col could do the trick.

Best,
Ulrike