Project rebranding

Since the beginig of the times this project's name has been "Portainer Stack Utils", which describes what it is and what it does. In order to avoid possible future legal issues with Portainer registered trademark it should be renamed to something that does not include "Portainer". Also, it has evolved to include more than just "stacks", so the name should not limit to "Stack Utils". Whatever name we come up with, it should be reflected by the executable's name (currently `psu`), which should keep short and easy to type.

**Proposals are welcome**