Bool is a convenience wrapper for boolean flag that picks its default value from the environment variable.

Int is a convenience wrapper for integer flag that picks its default value from the environment variable.

MustBool handles error (if any) returned by Bool according to the behaviour configured by `flag.CommandLine.ErrorHandling()` by either ignoring it, exiting the process with status code 2, or panicking.

MustInt handles error (if any) returned by Int according to the behaviour configured by `flag.CommandLine.ErrorHandling()` by either ignoring it, exiting the process with status code 2, or panicking.

String is a convenience wrapper for string flag that picks its default value from the environment variable.