Idea: DUCT_METADATAFILE option/env var handling

Inspired by
- #297

There could be `DUCT_METADATAFILE` to point to the .json (or .yaml?) file to be loaded upon completion of the CMD to be included under `metadata` attribute?

This is potentially for the complimentary effect to `DUCT_METADATA_*` of #297 of potentially the CMD itself (not just a wrapper) to inform `duct` on metadata to add e.g. for
- important to CMD versioning details (thus a potential solution for https://github.com/con/duct/issues/31 ?)