Checklist

• Are you running the latest v3 release? The list of releases is here.
• Did you perform a search about this feature? Here's the GitHub guide about searching.

What problem does this solve?

Following features would be useful to get a full featured release

• Add Required Flag tag Feature: Add required flag support #40
• Add support for Viper Add support for Viper #6 #
• Add support for map values Add map values #7
• Add support for multiple env vars Feature: Add support for multiple env #41
• Flag Type for setting date using natural language Add flag type that date from natural language #8 (https://github.com/olebedev/when)