Add support for executing operation upon receival of specific signal

[fritzrehde]

This would make watchbind even more customizable.
One use-case I thought of: maybe the watched command should be reloaded everytime a file is changed/edited inside the current directory. Instead of building this functionality into watchbind (which would require a lot of duplicated effort from other projects), one could spawn a separate file-watcher and then notify watchbind using signals once something changes.

[fritzrehde]

I think this shouldn't be restricted to just reloading the watched command on receival of a signal. Instead, we should generalize and allow executing any operation on receival of a signal. Reloading the watched command would then still be possible with a simple reload as the operation to be executed.