Add go module configuration

[rlskoeser]

This adds a go module so this theme can (I hope) be installed as a module. First step towards addressing #43

I created it by running:

hugo mod init github.com/dh-tech/hugo-bibliography

I think that once it is pushed to github on the main branch, it can be installed as a go module.

[rlskoeser]

It was already installable without this file, my error was caused by an incomplete module name. I got it working as a module + theme, I can suggest documentation changes with instructions - but the script is not included when installed as a module, so that part bears some more thinking on.