Finish removing Omnibot.State functions

Omnibot.State shouldn't be used anywhere anymore except as a GenServer
being started up in the supervisor.

Also, configuration must be loaded through Config.load/1 rather than
being constructed, because everything expects a tuple of {plugin,
config} now.

Finally, Omnibot.Core must be added to the configuration in order for
basic functionality.

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

omnibot.example.exs

@@ -7,6 +7,7 @@ config = %Config {
   ssl: false,
 
   plugins: [
+    Omnibot.Core,
     {Omnibot.Contrib.OnConnect, commands: [
       ["privmsg", "nickserv", "register", "password123", "omnibot@omni.bot"],
       ["privmsg", "nickserv", "identify", "password123"]