Pass extra GFA segment / link attributes (tags) to the visualization

[fedarko]

#23 describes doing this for depth, but there are other optional fields worth thinking about (k-mer coverage, link "mapping quality," etc.) -- see https://github.com/GFA-spec/GFA-spec/blob/master/GFA1.md.

This is a similar (but probably easier) issue to #147. I guess the ideal way to handle this is detecting all optional fields of all segments / links (with the understanding that only some segments or links in a file may have information for a given field), and then just passing these on to the visualization.