Aliases and multishell support

[Schniz]

This PR adds the ability to use a node version per-shell:
each fnm env --multi creates a temporary symlink and adds it to the $PATH
env var. fnm use will alter this temporary symlink instead of ~/.fnm/current.

It also adds the ability to create and update aliases (but not removing them):

fnm alias v8.11.3 some-name
fnm use some-name # just like `fnm use v8.11.3`