Optionally display multiple outline overlays

[d33bs]

There are datasets which include multiple outline types (for example, cell vs nuclei). Currently we enforce someone to select just one type of outline for overlay. For these scenarios it might be nice to somehow enable one to show multiple overlays (in different colors and/or textures) within the same image to maximize the amount of information one could glean from looking at the CytoDataFrame output within a Jupyter notebook.

Came up in conversation with @jenna-tomkinson .