Async Plugin Support

[half-duplex]

Requested Feature

As a plugin developer I would like it to be easier to take advantage of async features in my plugins.

• Support async setup(), or at least have the event loop already running
• Support async callbacks, probably transparently with iscoroutinefunction()

This also makes things like sopel-http and sopel-github (bottle) less complex to implement.

Problems Solved

Right now I have to use threads and start my own event loop if I want to use async features, which is unnecessary, annoying, and adds additional overhead and fragility.