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_supervisor.ex

@@ -15,7 +15,7 @@ defmodule Omnibot.PluginSupervisor do
 
     # These are plugins that need to be loaded for core functionality of the bot
     core = [
-      Omnibot.Core
+      Omnibot.Core,
     ]
 
     # Map the plugins in the configuration to the children