Is your feature request related to a problem? Please describe.
Sometimes it can be difficult or impossible to pass multiple files with config fragments. yaml support multiple documents in one file and safe_load_all from pyaml api loads that accordingly. It is standard yaml feature, it would be nice to support it and make in usable in cases when passing one file (composited from more files) would be easier.

Describe the solution you'd like
Support safe_load_all as yaml loader.

Describe alternatives you've considered
Passing multiple files will do the work, however it doesn't have to be always straightforward.

Additional context
I have prepared a patch