Improve Loader._traverse()
Improve Loader._traverse()

- Rename `visit_func` arg to `visit` (because the latter is just as
  meaningful but shorter)
- Swap order of `args` and `last_only` keyword args in def (because
  the `args` go with the `visit` func)
- Pass the next segment name to the `visit` function (could be useful)
- Allow `visit` function to replace next object by returning a value
  (esoteric; maybe useful)
- Return the item traversed to (could be useful; also, originally this
  was the reason passing a `visit` function wasn't required)
- Improve docstring

wylee/django-local-settings,PSU-OIT-ARC/django-local-settings