Slightly leaky abstraction -- figure out the best way to extend this

[elijahbenizzy]

This is for an extension of hamilton called hamiltime. Currently it's perfectly abstracted away except for this enum. Let's figure out how to make it a completely separate product.

https://github.com/stitchfix/hamilton/blob/c86ddd2adcc6d5812c8d1c769e76e72c1b06a580/hamilton/node.py#L20

Some ideas:

1. Release hamilton with hamiltime
2. Have this be an extensible class that hamiltime can overwrite
3. Have the function_graph, not the node type know about the node sources
4. ...