Allow per-clip and per-track colors

[jminor]

This begins to address #11

For now, this stores colors in a metadata field: "raven": {"color": "CYAN"}

Here's what it looks like.

[meshula]

Our Marker schema has a color property (with the enumeration you're using here). I'm wondering if it's time to talk about migrating the property higher in the schema, so that it becomes available to any composable element?

[timlehr]

Very cool! I wonder if this should be the beginning of discussion about a custom schema for Raven specific metadata.

[jminor]

Discussion about OTIO schema for per-clip colors can continue here: https://github.com/AcademySoftwareFoundation/OpenTimelineIO/issues/1614