Add basic template customisation

[stufisher]

This is a very simple method to allow templates to be customised, you maybe want something more complete in the long run, but this just lets you inject a custom template directory via an environment variable CUSTOM_TEMPLATE_PATH. It will fallback to the default location.

[MarcoMuellner]

Hey @stufisher, this is pretty neat! However, if we add this, there should be a part in the documentation that explains this. I also don't like the idea of having an environment variable, rather it should be an explicit option in the CLI interface. If those things are added, i'll gladly merge this :) Thanks for the contribution!

[stufisher]

I had to update the pre-commit-hook isort version for compatability with a newer version of poetry (https://stackoverflow.com/questions/75269700/pre-commit-fails-to-install-isort-5-11-4-with-error-runtimeerror-the-poetry-co)

Mostly refactored as requested, seems to work

[MarcoMuellner]

Great, thank you for your contribution :)