Functions that add edge(s) should optionally take label(s) for those edge(s)

[wilfwilson]

Currently these are separate steps. But I think that (for convenience and efficiency reasons) these should be able to be combined into a single step.

An incomplete list of the possible new interfaces that I envisage are:

DigraphAddEdge(D, edge, label)
DigraphAddEdge(D, src, ran, label)
DigraphAddEdges(D, edges, labels)

This idea came from thinking about #500.

[cwills15]

I'm interested in this - I'll take a look.