Finish migration to using a module to control core behavior

Previously, core behavior was handled in the Omnibot.Router module.
However, since the module system is robust enough, this work has been
delegated to the Omnibot.Core module.

Additionally, the Omnibot.State module has been augmented to keep track
of the modules that have been currently loaded, and methods that are
used to get the list of loaded modules and routing information for those
modules are coming from Omnibot.State as well (as opposed to
Omnibot.Config). This is to avoid requiring the Omnibot.Core module be
included in the config, plus avoiding modification of the runtime config
after it has already been loaded.

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

lib/core/join.ex

@@ -1,11 +0,0 @@
-defmodule Omnibot.Core.Join do
-  use Omnibot.Module
-  alias Omnibot.State
-
-  def on_join(channel, nick) do
-    cfg = State.cfg()
-    if nick == cfg.nick do
-      State.add_channel(channel)
-    end
-  end
-end

lib/core/welcome.ex

@@ -1,11 +0,0 @@
-defmodule Omnibot.Core.Welcome do
-  use Omnibot.Module
-
-  require Logger
-
-  @impl true
-  def on_msg(irc, _) do
-    Logger.info("Syncing channels")
-    Irc.sync_channels(irc)
-  end
-end