Environment variables and flags

Set flags using the standard library flag package, and complement them with environment variables.

• All Go's standard flag functions are used as normal and parse errors are returned as expected.

• Flags, if set, take precedence over the environment.

• Environment variables can have custom prefixes and suffixes.

• Usage output can be displayed with the standard -h flag.

• Usage output can be annotated with the configured environment variables.

fs := flag.NewFlagSet("test", flag.ContinueOnError)
fs.String("db", "", "Database thing")
_ = envflag.ParseFlagSet(fs, envflag.Prefix("APP_"), envflag.UsageSuffixer())

produces the following usage

Usage of test:
  -db string
        Database thing [APP_DB]