Incorporate existing pyproject.toml file

[diazona]

If a project already has a pyproject.toml file, we should incorporate its contents in the one we generate. TBD how to handle conflicting attributes - we have to figure out what setuptools does in that situation and replicate the behavior.

This has been in the back of my mind for a while but I was reminded of it by the discussion in #178. (thanks @astrojuanlu)