Consider switchting to an actively maintained envconfig package

[tillkuhn]

https://github.com/kelseyhightower/envconfig seems to be no longer maintained, https://github.com/sethvargo/go-envconfig looks similar, has 800+ starts and is still active, but does not support usage display out of the box. https://github.com/caarlos0/env hast +4k starts and also looks good, and mentions at least https://github.com/g4s8/envdoc as a related fully compatible project to generate documentation

See also discussion @ kelseyhightower/envconfig#204 (comment)