Differentiate between setting "global" and "local" versions

As far as "local" version setting, rgbenv's already got `exec`. Meanwhile, `use` and `no-use` are "global".

But there's some use for setting a "shell-wide" version, like pyenv's `shell`. Perhaps a `shell-use` and `shell-no-use` (very good names /s) command may be warranted... This might also need a `shell-init` command for both commands to be usable.