playlist/registry: add option "as_directory"

This allows users to disable the "CUE files as directories" feature
without having to disable the CUE playlist plugin completely.  This
feature has been annoying some users.

doc/plugins.rst

@@ -1231,6 +1231,8 @@ asx
 
 Reads :file:`.asx` playlist files.
 
+.. _cue_playlist:
+
 cue
 ---
 Reads :file:`.cue` files.

doc/user.rst

@@ -500,6 +500,11 @@ The following table lists the playlist_plugin options valid for all plugins:
      - The name of the plugin
    * - **enabled yes|no**
      - Allows you to disable a playlist plugin without recompiling. By default, all plugins are enabled.
+   * - **as_directory yes|no**
+     - With this option, a playlist file of this type is parsed during
+       database update and converted to a virtual directory, allowing
+       MPD clients to access individual entries.  By default, this is
+       only enabled for the :ref:`cue plugin <cue_playlist>`.
 
 More information can be found in the :ref:`playlist_plugins`
 reference.