Marker RGB colors

[jminor]

OTIO Markers have a "color" attribute which is a string taken from a list of color names ("RED", "GREEN", "YELLOW", etc.) However, some systems specify named colors that are not in this list, or allow arbitrary RGB values. In order to support a wider set of marker colors, we could switch OTIO to use an RGB triple like (255, 127, 0) or (1.0, 0.5, 0.0) and include helper functions to convert to and from lists of color names.

See prior work & discussion here: https://github.com/AcademySoftwareFoundation/OpenTimelineIO/issues/27

To limit the scope of this a bit, let us assert that markers never have a transparency/alpha, and that marker colors are not managed through colorspaces (sRGB, etc.). If anyone can show a counter example in an existing application, format, or workflow, please post it here.