extend `to_diff()` to junctions

[gpertea]

The to_diff() function is great for showing differences between exons, but sometimes it is very useful to highlight differences between introns and individual splice sites.

Not sure if it wouldn't make more sense to make it a dedicated to_jdiff() helper function for this , since I can imagine this might be a bit more complex as we could be looking at highlighting per-splice site differences - i.e. highlighting alternate donor vs. alternate acceptor vs. both (wholly novel introns)

[abartlett004]

Similarly, it would be great if shorten_gaps() could be applied to junctions as well so that junction arcs line up with the rescaled exons/introns. I couldn't figure out any way to do this with the existing functions, but if there's a way, I'd love to know!