Shell completion

[RCasatta]

Would be cool to have shell completion for the nix installed bitcoin binaries! (also in nixpkgs bitcoin, but I started here)

This is just a draft for multiple reasons:

• on top of upgrade bitcoin package from nixpkgs #646 which was not right
• doesn't work (the nix build is successful but the resulting image hasn't shell completion for bitcoin-cli)
• it is missing shell completion for other bins
• it makes sense to have completion also withGui
• I am not sure targeting directly the build dir for finding completionion files is right

At the moment I am not going to work on this, however, I wanted to leave it here in case me of the future want to retry or someone else want to start from here

[prusnak]

I will gladly review any PR to nixpkgs when it is ready, just tag me in the PR description.

[erikarvstedt]

I've created an upstream PR.