Add --meta, --header and --footer-cli params for customisation

[gnunicorn]

The template is baked into the binary, which is fine, but doesn't allow for much customization. A simple way we could change that, is by allowing the user to provide additional files via --meta. --header and --footer to be included in the final result. These should also be pre-processed with markdown if their file extension indicates so. The cli param description should make that clear.