Feature request : soft edge matching

[rossduncan]

Currently the matcher requires edges to match exactly; i.e. if the rule has an (un)directed edge then it can only match against an (un)directed edge in the graph.

For some applications it would be useful if an undirected edge in the rule could match against either a directed or undirected edge. This should an option that the user could switch on or off to avoid breaking existing projects.