Feature request: Support for GFA v1.1

[danydoerr]

The minigraph-cactus pipeline for next generation of pangenome graphs from the Human Pangenome Reference Consortium will no longer use P-lines but only W-lines to encode paths through the graph. From my perspective of the direction in which the GFA format is going to evolve, W-lines are here to stay, they will never be abandoned, same goes for P-lines. So, it would be great if rs-handlegrpah could support both.