Support different Bot Engines

[colestrode]

It would be great to support other engines. The first I'd like to look at is Slapp.

There are a lot of details to flesh out, and this would most certainly be a major release. This ticket is a placeholder, more details to come.

What would be ideal is for Skellington to have it's own Engine interface and then we could write different engine modules (e.g., skellington-engine-botkit, skellington-engine-slapp). This would keep Skellington from growing unwieldy and also allow others to write their own engines if they want. This may be a pie in the sky dream... but 🤞

I think the plugin API should support a "type" or "engine" field. If absent, it will default to "botkit" for backwards compatibility. Plugins whose type doesn't match the engine will not be loaded and an error should be logged.