Send script info to stderr

[homburg]

Currently dum outputs which commands it's going to execute, eg on

dum some-package-script

I'd like to be able to pipe the script output to something else easily without including the dum messages, eg.

dum some-script-producing-json | jq .

I'd suggest to either

1. Print dum own messages to stderr by default
2. Detect a tty/pipe situation and avoid printing messages (or use stderr in that case)

I'd be happy to make a pull request if you're interested