Support alarm discriminators

[fhunleth]

Currently Alarmist only supports atoms. Discriminating alarms of the same type, but applying to different things is possible by creating new atoms. I.e., NetworkDown.eth0. It would be far better to allow for at least 2-tuple alarms with the discriminator in the second slot. I.e., {NetworkDown, "eth0"}.

This requires updates to allow 2-tuple alarms since they're currently ignored. Alarmist should also offer ways of subscribing to events based on the Alarm ID only, the discriminator only, or both. It also requires updates to synthetic alarms.

Allowing for structured alarm discriminators like %NetworkDownAlarm{ifname: "eth0"} seems nice to think about, but might be too much for the first pass considering that it can't be completely freeform and that it invites descriptive (non-descriminator) fields.