Support path annotations

[asl]

@ekg Will it be possible for you to draft requirement here? Something more specific than just "use coordinates along the path", if anything?

[ekg]

Sure! The basic idea is that any path range, e.g. BED interval in a FASTA sequence space corresponding to the path space of the graph, can be converted into a walk on the graph. This will then overlap a series of nodes. It would be great to be able to color the sub-path defined by a BED interval a consistent way based on the annotation name (such as a hash of the name) and to add a text lable correspond to the annotation name or fourth column of the BED.