Adding "extra user data"

[rocktavious]

Would you be open to accepting a PR to add an option that adds extra user data script section.

The reason I want to do this is because in our company we use https://tailscale.com/ but as our VPN but also as a mechanism to SSH into our instances.

This extra option would allow me to inject the ability to install and run tailscale on the box.

Thanks for your consideration.