Useful in combination with `--bare` to list just the unique version
numbers without the extra directory entries that are symlinks to other
version numbers in the same directory.
Docs are comprised from "Usage", "Summary" and "Help" sections, where
"Help" can span multiple commented lines. If it is missing, "Summary" is
shown in its place.
References #204, references #206
`rbenv-versions` tries to read the current version to display a marker
next to it, but if that fails the whole script aborts.
This change makes it so that the failures from `rbenv-version-name` are
tolerated. It also makes the `--bare` mode never call it in the first
place, because it doesn't need to display a marker.