compose-tracker: add main for local testing/manual runs
    
    Allows for local testing or also manually triggering a run.
    You can manually trigger a run by passing in a URL to a datagrepper
    message like:
    
    compose_tracker.py 'https://apps.fedoraproject.org/datagrepper/id?id=2019-05494a6e-2619-4290-96d0-07d8804c3f2a&is_raw=true&size=extra-large'
    
    If you want an actual pagure issue to be opened then set PAGURE_TOKEN
    first:
    
    export PAGURE_TOKEN=abcd
    compose_tracker.py 'https://apps.fedoraproject.org/datagrepper/id?id=2019-05494a6e-2619-4290-96d0-07d8804c3f2a&is_raw=true&size=extra-large'