Differentiate between setting "global" and "local" versions

[ZoomTen]

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.