Add basic support for adding tags to Snappy reports.

A proper implementation requires westerncapelabs/django-snappy-vumi-bouncer#7, but we can at least add them to the message for now which allows them to be picked up with a Snappy trigger.
Maybe this can be included in the `_.defaults` above? Just so that everything is in the correct place, and we don't run into `0 || '-' === '-'`