Make fc2-live-dl emit events to autofc2

[hizkifw]

Right now, autofc2 is only able to wait for fc2-live-dl to record a stream and exit. It would be cool if fc2-live-dl can emit events such as when the stream has started recording, when it's finished and is muxing, etc.

This would make it possible to add integrations on top of autofc2 such as Discord notifications for when streams go live. It would also improve config hot-reloading, since autofc2 can safely recreate the fc2-live-dl instance when it notices the config changed and the instance isn't doing anything.