input: Regexp type for kelseyhightower/envconfig integration

[kamilsk]

Motivation: extend flag types.

Draft

type Regexp regexp.Regexp

func (reg *Regexp) Decode(value string) error {
	raw, err := regexp.Compile(fmt.Sprintf(`^%s$`, value))
	if err != nil {
		return err
	}

	*reg = Regexp(*raw)
	return nil
}

func (reg *Regexp) Matcher() *regexp.Regexp {
	return (*regexp.Regexp)(reg)
}

See https://github.com/kelseyhightower/envconfig#custom-decoders.