unnumbered directive for toctree

[zerothi]

Is your feature request related to a problem? Please describe.
When having a numbered toctree, the subtrees will inherit this setting.
However, in some cases one might wish to remove the numbering at some level.

Describe the solution you'd like

I would like one to be able to do:

<file1.rst>

.. toctree::
    :numbered:
    file2


<file2.rst>

.. toctree::

   second level
   file3

<file3.rst>

.. toctree::
   :unnumbered:
   ...

This would be ideal since one could automatically turn on/off the numbering.
Some levels of my toc might have many entries where the numbering is obfuscating the list, where others have short lists and thus might be requiring the numbering.