add get_edges_between(from, to) method to Graph object

[davewhiteland]

Currently there's no built-in way to find all the edges between to vertices and it feels like the Graph ought to be able to do that. (It would handle the wrinkle of bi-directional edges too).

Maybe get_edges_between(from_vertex, to_vertex) returns an array of edges (possibly empty) but...

get_edge_between(from_vertex, to_vertex [,chooser]) returns just one (possibly null) — in graphs where there are no duplicate edges, having to handle the array will be an annoyance. There's already a chooser defined in the GraphVertex.get_edge_to(to_vertex, chooser) method.