Allow to specify edge metadata

[jonnor]

For instance for capacity (buffer bounds)

[bergie]

From JPM:

In the pre-NoFlo .fbp notation, connection capacity is specified by a number in brackets after the arrow.

One question is how to specify unbounded buffers.

[jonnor]

I think special-casing capacity is not worth it. Think it should be generic edge metadata (key,value) instead.

0 can mean unbounded.

[jonnor]

Semi-related, we may want generic key,value metadata for the graph also, like for #33 and #27