Option to recursively merge content of multiple config.yml files

[NovaNekmit]

Currently, merge! is used for every supplied config file, which works good for most things, but "appending" to treat_as or unity_helpers won't work, which prevents effective composing.

Another command line flag (maybe -m for "merge", or -a for "append"), which recurses into the the options to append arrays and merge hashes (instead of overwriting) would help with that.

Alternatively, a special value in the "overlay file" that the merge logic interprets as "keep existing values" would also work.

For context, I am working with a modular project and want to avoid "leaking" types into other modules.

My current workaround is supplying --unity-helpers=shared/unity_helpers.h;module_a/unity_helpers.h as command line argument, and using macros inside the unity_helper in place of treat_as.