[Feature request] Run command on image copy

[jerbaroo]

Similar to #119 but providing a command to be run on image copy rather than image save, either via config file or via CLI args.

This would allow emitting a notification if I have copied to clipboard. This would be nice in combination with #119, to have different notifications in the two different success scenarios.

Example: swappy --on-copy 'notify-send "Copied to clipboard"' ...

[jerbaroo]

Ability to have some kind of feedback on image copy/save would be a positive improvement to UX.