Remove Plugin.Agent, replace everywhere with Plugin.GenServer

Being able to handle arbitrary messages from other processes is a
generally useful thing to have for fine-grained control of the plugin.
Agents don't provide that, while GenServers do. I've removed all
instances of the Agent and replaced it with the GenServer and everything
appears fine so far.

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

lib/plugin/plugin.ex

@@ -2,7 +2,7 @@ defmodule Omnibot.Plugin do
   defmacro __using__([]) do
     quote do
       use Omnibot.Plugin.Base
-      use Omnibot.Plugin.Agent
+      use Omnibot.Plugin.GenServer
     end
   end
 end