It's possible for plugins to define new asdf commands. This way plugins can extend asdf capabilities or expose utilities related to their managed tool.
For example, a `foo` plugin might expose the command `asdf foo bar` by providing an executable file at `bin/bar`.
If `bin/bar` is a file but has no executable bit set, then its considered a source-able bash script, and will be sourced
with all the functions in `$ASDF_DIR/lib/utils.sh` already loaded.
A good example of this feature is the `nodejs` plugin, where people must import the release team keyring before
installing a nodejs version. People can execute the following command without having to know where exactly is the plugin located.
