Allow to specify the settings you want to see

Now, print_settings accepts settings variable names and just print
those variables:

$ python manage.py print_settings DEBUG MEDIA_URL
DEBUG                                    = True
MEDIA_URL                                = '/media/'
$

or:

$ python manage.py print_settings DEBUG MEDIA_URL --format=pprint
{'DEBUG': True, 'MEDIA_URL': '/media/'}
$