Finish up Plugin.Supervisor, replace markov and wordbot implementations with it

Both markov and wordbot have some auxiliary processes that run to keep
track of things. Previously, they both had custom supervisors grafted on
to the Plugin.Base - now, this grafting is automated with
Plugin.Supervisor.

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

lib/plugin/base.ex

@@ -79,6 +79,7 @@ defmodule Omnibot.Plugin.Base do
     end
   end
 
+  #@callback handle_msg(irc :: pid(), msg :: %Omnibot.Irc.Msg{}) :: any
   @callback on_msg(irc :: pid(), msg :: %Omnibot.Irc.Msg{}) :: any
   @callback on_channel_msg(irc :: pid(), channel :: String.t(), nick :: String.t(), line :: String.t()) :: any
   @callback on_channel_msg(