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

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

lib/plugin/agent.ex

@@ -0,0 +1,40 @@
+defmodule Omnibot.Plugin.Agent do
+  defmacro __using__([]) do
+    quote do
+      alias Omnibot.Plugin
+      use Agent
+
+      def start_link(opts) do
+        cfg = opts[:cfg]
+        state = opts[:state] || on_init(cfg)
+        Plugin.Agent.start_link(cfg, state, opts)
+      end
+
+      def cfg, do: Plugin.Agent.cfg(__MODULE__)
+      def state, do: Plugin.Agent.state(__MODULE__)
+
+      def update_state(update, timeout \\ 5000),
+        do: Plugin.Agent.update_state(__MODULE__, update, timeout)
+    end
+  end
+
+  def start_link(cfg, state, opts) do
+    Agent.start_link(fn -> {cfg, state} end, opts)
+  end
+
+  def cfg(agent) do
+    Agent.get(agent, fn {cfg, _} -> cfg end)
+  end
+
+  def state(agent) do
+    Agent.get(agent, fn {_, state} -> state end)
+  end
+
+  def update_state(agent, update, timeout \\ 5000) do
+    Agent.update(
+      agent,
+      fn {cfg, state} -> {cfg, apply(update, [state])} end,
+      timeout
+    )
+  end
+end