Finish up Plugin.Supervisor, replace markov and wordbot implementations with it

Both markov and wordbot have some auxiliary processes that run to keep
track of things. Previously, they both had custom supervisors grafted on
to the Plugin.Base - now, this grafting is automated with
Plugin.Supervisor.

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

lib/plugin/supervisor.ex

@@ -0,0 +1,64 @@
+defmodule Omnibot.Plugin.Supervisor do
+  @default_opts [include_base: true, opts: [strategy: :one_for_one]]
+
+  defmodule CfgState do
+    use Agent
+
+    def start_link(opts) do
+      {cfg, opts} = Keyword.pop(opts, :cfg)
+      {state, opts} = Keyword.pop(opts, :state, nil)
+      Agent.start_link(fn -> {cfg, state} end, opts)
+    end
+
+    def cfg(pid), do: Agent.get(pid, fn {cfg, _} -> cfg end)
+
+    def state(pid), do: Agent.get(pid, fn {_, state} -> state end)
+
+    def update_state(pid, fun, timeout \\ 5000),
+      do: Agent.update(pid, &{&1, apply(fun, [&1])}, timeout)
+  end
+
+  defmacro __using__(opts) do
+    opts = opts ++ @default_opts
+
+    quote do
+      use Supervisor
+      use Omnibot.Plugin.Base
+
+      ## Client API
+
+      def start_link(opts) do
+        Supervisor.start_link(__MODULE__, {opts[:cfg], opts[:state]}, opts)
+      end
+
+      def cfg() do
+        Omnibot.Plugin.Supervisor.CfgState.cfg(__MODULE__.CfgState)
+      end
+
+      def state() do
+        Omnibot.Plugin.Supervisor.CfgState.state(__MODULE__.CfgState)
+      end
+
+      def update_state(fun) do
+        Omnibot.Plugin.Supervisor.CfgState.update_state(__MODULE__.CfgState, fun)
+      end
+
+      ## Server callbacks
+
+      @impl Supervisor
+      def init({cfg, state}) do
+
+        base_children = [
+          {Omnibot.Plugin.Supervisor.CfgState, cfg: cfg, state: state, name: __MODULE__.CfgState},
+        ]
+        children = 
+          (if unquote(opts[:include_base]), do: base_children, else: []) ++ children(cfg, state)
+        Supervisor.init(children, unquote(opts[:opts]))
+      end
+
+      @behaviour Omnibot.Plugin.Supervisor
+    end
+  end
+
+  @callback children(cfg :: [atom: any], state :: any) :: [atom | {atom, [atom: any]} | {atom, any, [atom: any]}]
+end