Provide the possibility to configure which tags to propagate using a yaml file

Hi,

These commits add some testing on calling the application and where primarily used to make sure that I don't change any behavior from the application.  In order to made it easier testable I refactored cli.py a little bit so that mocking would be more easy.  Other than that the first commit doesn't implement real changes.  The 2nd commit adds the possibility to take a configuration file to define the tags that are to be propagated.  This allows people to use graffiti-monkey for any tags they wish.

Kind regards,

Peter