Fixed #12347 -- Added a mimetype option to sitemaps views to further customize...

Fixed #12347 -- Added a mimetype option to sitemaps views to further customize the output of the view, e.g. to support Yahoo's plain text urllist format. Thanks, afurlan.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16475 bcc190cf-cafb-0310-a4f2-bffc1f526a37