Allow overriding pipeitem/livepipeitem (or add text to edges)

[dberlin]

Right now i have a fork where i've added support for adding text to edges (the annoying part being the transforms to properly rotate the text to match the rotating of the edge lines, without hitting the node direction indicator :P).

I do this right now as a direct edit of PipeItem.

Another way to do it would be allow for overriding the default pipe class that gets used when creating pipes.

i"m happy to work on a PR that does either -

1. Add support for edge text annotations to the existing pipeitem class
   or
2. Create a PipeItem factory that gets used to create PipeItems

I don't think it needs the full generality of node item factories, and happy to just add support for edge text instead.

Do you have a preference?