Add sushi-override

Consider supporting something like sushi-config-override.yaml, containing values that will supersede those in sushi-config.yaml.
this file could be gitignored.
The use case:
When we need to do some changes, e.g. when we prepare a release candidate, we need to adjust the sushi-config.yaml. These changes are simple: adjust version, status,...
Some of them may be merged again to master, some of them not.

discussion in
https://chat.fhir.org/#narrow/channel/215610-shorthand/topic/sushi.20config.20override

Open:
What this would do with nested or repeating elements? I would start with complete override: if i enter an element in the override, all of that element replaces all that what was in the original yaml.

Start by locating the entry point that reads sushi-config.yaml, then review the linked chat discussion about sushi-config-override.yaml and the open question about nested or repeating elements. Done means an override file can supersede the base configuration, with the complete-replacement behavior for nested elements clarified and covered by tests.

Written by the indexing model from the issue text.