Support `arrow` for `geom_linerange()` and `geom_pointrange()`

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.