Feature Request: Consider feature/capability to allow sourcing from file(s) in addition to getenv()

[nneul]

Seems like a number of go based apps and containers are leveraging this module. It could potentially be very useful if it had the ability to source from a file in addition to env vars in a standardized manner -- without the container/app author needing to explicitly do that themselves.

This would particularly be useful with leveraging k8s secret mounts with unmodified container images where you want to read as files and not as environment variables.