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/supervisor.ex

@@ -0,0 +1,28 @@
+defmodule Omnibot.Plugin.Supervisor do
+  use Supervisor
+
+  def start_link(opts) do
+    {plugin, opts} = Keyword.pop(opts, :plugin)
+    {cfg, opts} = Keyword.pop(opts, :cfg)
+    start_link(plugin, cfg, opts)
+  end
+
+  def start_link(plugin, cfg, opts) when is_atom(plugin) do
+    Supervisor.start_link(__MODULE__, {plugin, cfg}, opts)
+  end
+
+  def child_spec(arg) do
+    id = Module.concat(arg[:plugin], Plugin.Supervisor)
+    %{
+      id: id,
+      start: {__MODULE__, :start_link, [arg]},
+    }
+  end
+
+  @impl true
+  def init({plugin, cfg}) when is_atom(plugin) do
+    state = plugin.on_init(cfg)
+    children = IO.inspect(plugin.plugin_children(cfg, state))
+    Supervisor.init(children, strategy: :one_for_one)
+  end
+end