Create get-ghcup.sh, fixes #36

This follows the style of the rustup installation script that has a friendly welcome message, tells you what the script is going to do, and then asks if you'd like to proceed. This is meant to be a one-command setup of everything you need to get started with Haskell.

I already had this written so adding it for comparison to #40. Would still need to update README and clean a few things up. Let me know what you think!