Fixed #22257 -- Added file output option to dumpdata command.

Added ``-o`` or ``--output`` to dumpdata for specifying filename to which the data would be written.
default still ``stdout``.
People may prefer this on case of small databases.
Also its backwards compatible

Refs.
- https://code.djangoproject.com/ticket/22257
- https://code.djangoproject.com/ticket/22251