I'm not sure why it started to happen but the installation fails in a
high percentage. The installation process itself isn't that hard and I
think that dropping the automation would be better right now.

If there's a good way of automating it, I'd be happy to revert the
change — but I don't install new Macs this often.

Another cool thing we can do is implement a `fnm install-env` or
`fnm env --install`, along with `fnm doctor` that would spit out every
environment variable and information we will need for opening an issue.

* make shell installation optional
* add --without-shell-config to fix #1

* Create fnm.yml

* test

* test

* move stuff to install