`fnm upgrade` command

It would be nice to have an fnm upgrade command that would upgrade the current Node version to the latest minor version, and optionally remove the old one (e.g. with an extra CLI option, like --rm-old).

Alternatively, if upgrading minors is considered too dangerous to be the default, this command could upgrade just the patch version, and upgrading the minor version would be done by explicitly passing a separate CLI option (e.g. fnm upgrade --minor).