Add ActiveJob support for subscriptions

[rmosolgo]

Currently, you could implement background jobs for calling trigger, which is great, but I think this gem could include built-in support à la Rails's deliver_later. I'll take inspiration from that method here.

TODO:

• [x] Add .trigger_later
• [ ] Add some way to put execution and/or delivery in the background too. This would add resilience to systems sending payloads over the wire.