Support arrow for geom_linerange() and geom_pointrange()

[teunbrand]

The idea here is that we can put in a cheeky errorbar through the arrow argument. (with 90 degree angles). It has two possible upsides:

1. It avoids having to draw geom_errorbar() first with geom_point() on top.
2. You can size the 'hooks' on the ends absolutely: you no longer need to know the data resolution or anything of the sort.