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)