Multi-shell support

[Schniz]

I believe this can be a breaking change, therefore should mark a version 2?

In its core, the following thing should work:

bash 1            | bash 2       
------------------|-------------
$ fnm use 11.0.0  |
Using Node 11.0.0 |
                  | $ fnm use 10.0.0
                  | Using Node 10.0.0
$ node -v         | $ node -v
11.0.0            | 10.0.0

How would that work?

I guess we can create a directory in /tmp, that will contain symlinks. A symlink will be created every time we call fnm env! The system will clean it up itself, so there is no problem there:

$ fnm env
export PATH=/tmp/fnm_environments/terminal_xxyyzz:$PATH

This may relate also to #6, because we might have to implement some kind of "aliases". This shouldn't be hard as well because - it's also a symlink. Isn't that great? Everything is a symlink!

[Schniz]

This can also be opt in - fnm env --local