Add a way to set a static tag

[algernon]

Adds the hook.SetTag function that makes it possible to pre-define a tag. This way a tag does not have to be included in every single message.

With this implementation, the static tag set on the hook overrides whatever may be in the message itself.

[coveralls]

Coverage decreased (-0.7%) to 83.444% when pulling 3c88cffdd386496566b758d474fa3edc25f4339b on algernon:f/static-tag into 12d8dbadcb812ae575268488f7c08320f3df763d on evalphobia:master.

[evalphobia]

Thanks!