Add fnm exec to run commands with the fnm environment

[Schniz]

Adds fnm exec to run a shell executable with the current node version (or a custom one provided):

fnm exec -- node -v # will print the current Node version in fnm
fnm exec --using 12 -- node -v # will print the version of the latest node 12 installed
fnm exec --using-file -- node -v # will print the version of the current directory's node version (based on `.nvmrc` or `.node-version`)