422 source response headers

[EyePulp]

This is a PR to address https://github.com/h2non/imaginary/issues/422.

Currently includes:

• added support for new cli arg for -source-response-headers
• added explicit handling for conflicts/fallbacks between -http-cache-ttl and -source-response-headers cache-control
• added reademe & usage documentation updates / examples
• added unit tests

Does not include a version bump, as I'm not sure how substantial the change is or the norms for versioning. Advice welcome.