Adding/removing plugins requires a restart

[dgw]

This is solely a limitation of how the plugin management stuff works. Specifically, when it is called during the bot's lifecycle.

Especially with our shiny new plugin machinery (#1479 and friends), it should be pretty doable to detect new plugins on demand and load them without having to restart Sopel.

Additionally, we should be able to remove all references to a loaded plugin and effectively disable it at runtime, without having to restart.

This assessment is backed by the plugin machinery's refactorizer himself:

<+Exirel> Detect new plugin: doable. Disable/enable plugin whitout restarting: totally doable yes.

[dgw]

De-milestoning for lack of planned work on plugin system. Remains on wishlist, as it would indeed be nice to load new plugins without a restart someday.