Make extract_stars able to take plain Numpy arrays

It would be nice to not have to create an NDData object just for ``extract_stars`` - other parts of photutils support plain Numpy arrays. Maybe ``extract_stars`` could include the ``@support_nddata`` decorator:

http://docs.astropy.org/en/stable/api/astropy.nddata.support_nddata.html

Locate the extract_stars entry point and review how other photutils functions use Astropy's support_nddata decorator. Check the linked support_nddata documentation, then verify that plain NumPy arrays and NDData objects are both accepted by extract_stars without changing its existing behavior.

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