Move watcher into a submodule; add mix task

lib/mix/tasks/pandoc.ex

@@ -0,0 +1,16 @@
+defmodule Mix.Tasks.Pandoc do
+  use Mix.Task
+
+  @out_dir "priv/static"
+
+  @impl true
+  def run([profile | _args]) do
+    IO.puts "profile: #{profile}"
+
+    "priv/posts"
+    |> File.ls!()
+    |> Enum.each(fn path ->
+      Pandoc.install_and_run(Path.join(["priv", "posts", path]))
+    end)
+  end
+end