custom: Option to disable running script at startup with signal set

[benjamb]

With a signal configured and interval unset, the custom module will still execute the script at startup.

While I could have my script write to a temporary file to save some kind of state, it would feel much cleaner if I could prevent the script from being run at startup, and will thus only run when triggered via the configured signal.

[benjamb]

Better yet, if the script were made aware of how it was triggered, that would likely be much more powerful. That could be by CLI args, stdin or environment variables, whichever makes the most sense.