Externally specified highlights

[ggonnella]

The idea is to indicate which parts of a segment e.g. align to a Blast query.

This can be also an alternative application of fragments. So the plan could be to implement a script which translates the Blast results into Fragment lines, with a color specified in the Gfaviz style tag.

An alternative is to allow to specify highlights in tags. However, using fragments is actually a good idea, as it gives a new application to fragments.