Clean up bash idioms, use modern interpolation style

[obscurerichard]

This PR seeks to clean up the bash idioms used in the alias script. It offers some additional help for debugging, it is possible to run this as DEBUG=true bash ./alias to get a line by line rendition of what it is trying to do, and if there are any errors.

It introduces some comments to the alias script that explain its usage, and links back to the documentation.

I have a mind to also make this shellcheck clean but wanted some feedback about whether PRs are welcome in this project before doing further work.

This script should be robust in the face of adding additional error checking, such as the Bash unofficial strict mode.

[rolfschr]

Oh, and please also feel free to add a line here.