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`)
```
Gal Schlezinger