Return stdout and stderr separately on success

[SaphiraKai]

Hi! I'm currently working on a little system utility that involves piping input into a dmenu clone (wofi) and reading the output from it to determine which choice the user made, but since shellout combines the stdout and stderr streams, I get all the GTK warnings etc. in the output as well, which throws off my parser.

For now I'll probably just set LetBeStderr to work around this, but that has some issues as well since it means it won't capture any error messages when the command fails, and that I have no control over the error messages printed to stderr in the terminal.

It would be super useful if shellout provided a way to extract stdout and stderr separately!