Add option to catch `stdout` as well as `stderr`

Some programs incorrectly output their error messages on stdout rather than stderr. While those programs themselves should probably be fixed, in some cases its not possible. It would be great to add an option to catch stdout!

(Thanks for fixing the others issues so promptly. Sorry to hear that you can’t work out how to enable PRs… I don’t have much experience with Bitbucket unfortunately so I’m not sure I have any solutions.)