Add a way to test the bot in a non-prod repo

[bfabio]

Right now the bot hardcodes the org and it can't really be used in another repo (fe. a personal one) to check everything is good after a new release, because it would notify real people and add a lot of noise.

Being able to test it in another repo would be nice. One solution could be having an env variable setting a dry run mode where the bot prints the actions / messages to stdout instead of performing them / commenting.