`node_color` cannot use `altair.Color` type

[HHammond]

👋🏻 I'm running into an issue when trying to draw a NetworkX graph with Altair.

I'm trying to render a DAG with colored nodes, but the color parameter does not accept alt.Color which would be useful for situations where not all categories have data.

The fix would be to update this if statement to accept the altair.Color type.

https://github.com/Zsailer/nx_altair/blob/master/nx_altair/draw_altair.py#L92-L94