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

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

lib/plugin/meta.ex

@@ -0,0 +1,37 @@
+defmodule Omnibot.Plugin.Meta do
+  defmodule Hooks do
+    defmacro __before_compile(_env) do
+      quote do
+        def children(_cfg), do: []
+      end
+    end
+  end
+
+  defmacro __using__([]) do
+    quote do
+      use Omnibot.Plugin.Base
+      use Supervisor
+
+      @behaviour Omnibot.Plugin.Meta
+
+      ## Client API
+
+      def start_link(opts) do
+        Supervisor.start_link(opts)
+      end
+
+      ## Server callbacks
+      def init(opts) do
+        cfg = opts[:cfg]
+        children = children(cfg)
+        Supervisor.init(children, opts)
+      end
+
+      defoverridable Omnibot.Plugin.Meta
+
+      @before_compile Omnibot.Plugin.Meta.Hooks
+    end
+  end
+
+  @callback children(cfg :: any) :: [{atom(), [{atom(), any}]}]
+end