Save metadata for vertices and edges

My use case is to build a Graph from data coming from a DB. When there are changes to it, I want to persist it back to DB.

For this to work, I am currently abusing the labels from vertices and edges. For vertex it is a Keyword list and for edges it is a map.

It would be much more convenient if there was a `metadata` field for both and a function to set it. Something like:
```elixir
Graph.put_metadata(g, v, %{id: 1234, comment: "some comment"}) # sets metadata for vertex
Graph.put_metadata(g, v1, v2, l, %{id: 1234, comment: "some comment"}) # sets metadata for edge
```

I can create a PR if this is a worthwhile feature.