Remove (supervisor) opts from Omnibot.Plugin opts, update Plugin.Supervisor to use :one_for_all strategy by default

Plugin opts no longer contains options for the supervisors that watch
plugins, and said supervisors now adopt the one_for_all strategy. This
is because these servers *generally* need one another to stay alive.

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

lib/plugin/supervisor.ex

@@ -22,6 +22,6 @@ defmodule Omnibot.Plugin.Supervisor do
   @impl true
   def init({plugin, cfg}) when is_atom(plugin) do
     children = plugin.plugin_children(cfg)
-    Supervisor.init(children, strategy: :one_for_one)
+    Supervisor.init(children, strategy: :one_for_all)
   end
 end