Add lib/plugin that was removed in the last commit, whoops

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

lib/plugin/gen_server.ex

@@ -0,0 +1,50 @@
+defmodule Omnibot.Plugin.GenServer do
+  defmacro __using__([]) do
+    quote do
+      alias Omnibot.Plugin
+      use GenServer 
+
+      def start_link(opts) do
+        cfg = opts[:cfg]
+        state = opts[:state]
+        GenServer.start_link(__MODULE__, {cfg, state}, opts)
+      end
+
+      def cfg() do
+        GenServer.call(__MODULE__, :cfg)
+      end
+
+      def state() do
+        GenServer.call(__MODULE__, :state)
+      end
+
+      def update_state(update) do
+        GenServer.cast(__MODULE__, {:state, update})
+      end
+
+      ## Server callbacks
+
+      @impl GenServer
+      def init({cfg, state}) do
+        state = state || on_init(cfg)
+        {:ok, {cfg, state}}
+      end
+
+      @impl GenServer
+      def handle_call(:cfg, _from, {cfg, state}) do
+        {:reply, cfg, {cfg, state}}
+      end
+
+      @impl GenServer
+      def handle_call(:state, _from, {cfg, state}) do
+        {:reply, state, {cfg, state}}
+      end
+
+      @impl GenServer
+      def handle_cast({:state, update}, {cfg, state}) do
+        {:noreply, {cfg, apply(update, [state])}}
+      end
+    end
+  end
+end
+