[Feature] Allow picking specific default file to merge with specific override file in API

[Ozair Patel]

My directory structure for my project looks like this

$ tree live/environments
live/environments
├── _default.yaml
├── dev.yaml
└── prod.yaml

I want to be able to selectively choose to merge dev.yaml with _defaults.yaml or prod.yaml with _defaults.yaml. Hell if I wanted to I should be able to merge dev.yaml with prod.yaml.

Expected Behaviour

# Non-existent API
config_processor.process_custom(default="live/environments/_default.yaml", override="live/environments/dev.yaml")

Merges dev.yaml on top of _default.yaml

Actual Behaviour

config_processor.process(path="live/environments")

Takes the last file, prod.yaml and merges with _default.yaml.

[Adrian Muraru]

IMO, this adds too much complexity.
"hi" in himl stands for hierarchical and in this case we'd introduce merging logic horizontally.
In your example above - the dev and prod could be moved as sub-folders and achieve similar behaviour