Allow custom template for AIP markdown

[reem]

This would be nice to allow adding e.g. additional disclaimers or other boilerplate into the template

Would also be nice to be able to (optionally) provide the summary and/or specification sections within the config in order to allow completely generating the markdown from the generator

[sakulstra]

I can see how this is useful, perhaps we could have a --footer "" cli parameter or similar.
Then teams can just add their "standard" template as a predefined package.json script (like we have with bgd labs https://github.com/bgd-labs/aave-proposals-v3/blob/main/package.json#L9)

provide the summary and/or specification sections within the config in order to allow completely generating the markdown from the generator

Will move this to a separate issue.
It's sth we have planned (and there's already some part implemented iirc)