Use shellcheck for the bash scripts

[rebornplusplus]

This project started out as a hacky bunch of scripts. (It still is hacky, sort-of.) But since this is being used seriously, we should take better care of the scripts.

Use shellcheck before pushing any script. I am thinking of configuring a pre-commit or pre-push git hook job and a CI job that runs shellcheck and other relevant linters to keep the scripts nice and tidy.

Ideas are welcome!