Finish plugin and routing overhaul, there's a new model in town:

* Plugins all derive from Omnibot.Plugin. There still is a base plugin,
  in case we want to have another plugin backend instead of a GenServer
* All plugins are monitored by a unique Plugin.Supervisor, which is in
  turn started by the PluginSupervisor (yes this is confusing, yes it
  needs to be renamed)
* Any other auxiliary child processes may be started through the
  Plugin.children/1 function.
* By default, plugins have a CfgState process which is an Agent that
  keeps track of the plugin's configuration and state
* Plugin API is now called through the GenServer backend for better
  synchronicity.
* Very few changes to the front-facing Plugin API, which is nice

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/plugin/base.ex

@@ -19,6 +19,11 @@ defmodule Omnibot.Plugin.Base do
       @impl true
       def on_init(_cfg), do: nil
 
+      @impl true
+      def on_msg(irc, msg) do
+        route_msg(irc, msg)
+      end
+
       @impl true
       def default_config(), do: @default_config
 
@@ -33,11 +38,6 @@ defmodule Omnibot.Plugin.Base do
 
       @behaviour Plugin.Base
 
-      @impl Plugin.Base
-      def on_msg(irc, msg) do
-        route_msg(irc, msg)
-      end
-
       defp route_msg(irc, %Irc.Msg {prefix: nil}), do: nil
 
       defp route_msg(irc, msg) do