Adds an edge between two vertexes and returns it.
If force-new? is true, the edge is added even if one already exists.
If the vertexes are not found in the graph, they will be added
(unless :error-if-not-found? is true). The class of the edge can be
specified using :edge-class or :edge-type. If :edge-type is used, it
can be either :directed or :undirected; the actual class of the edge
will be determined by using the edge-types of the graph. If
neither :edge-type nor :edge-class is specified, then a directed edge
will be created.

If-duplicate-do is used when the 'same' edge is added more than
once. It can be either a function on one variable or :ignore
or :force. If it is :ignore, then the previously added edge is
returned; if it is :force, then another edge is added between the two
vertexes; if it is a function, then this function will be called with
the previous edge.