Split geom_hline into geom and annotation (fixes #426)

This fixes #426. As discussed, it splits `geom_hline` into a geom and annotation.

If this looks like the right approach, I can do the same for vline and abline. If so, should I put `annotate_hline`, `annotate_vline`, `annotate_abline` all in one file since they're quite simple? Or they could be added to their respective `geom-Xline.r` files.

Also, some of the visual tests will have to be updated.