Added ability to choose which attributes get passed to the dot graph node/edge.

I've added the ability to filter out or add additional keys to the attributes of a dot node or a dot edge. 

This is useful as some data is not really part of the loom node/edge but it is meta-data like shape, colour etc. This gives the user a way to add that information if needed.

I've also added a way to add some metadata for the global dot nodes and edges.