bug#41868: [PATCH] Add project-clean-up command

[Dmitry Gutov]

On 16.06.2020 20:12, Philip K. wrote:
> It sounds good, so I used it in the revised patch below, together with a
> few other minor improvments which Basil mentioned.

Thank you, I pushed with some minor changes.

- Docstring further rephrased based on Basil's suggestion.
- The variable renamed to project-kill-buffers-skip-conditions, hope you 
don't mind.

Should we add a key binding for it as well?