feat: Multiple Required/Persistent Flags

[SeanPino]

This adds the ability to call MarkFlagsRequired and MarkPersistentFlagsRequired for n flags instead of one by one.

Flags:

rootCmd.Flags().StringVarP(&Region, "region", "r", "", "AWS region (required)")
rootCmd.Flags().StringVarP(&Failover, "failoverRegion", "f", "", "AWS failover region (required)")
rootCmd.MarkFlagsRequired("region", "failoverRegion")

Persistent Flags:

rootCmd.PersistentFlags().StringVarP(&Region, "region", "r", "", "AWS region (required)")
rootCmd.PersistentFlags().StringVarP(&Failover, "failoverRegion", "f", "", "AWS failover region (required)")
rootCmd.MarkPersistentFlagsRequired("region", "failoverRegion")

This is similar to grouping but doesn't have the drawback of needing any members of the group to be set just all are required.

[CLAassistant]

All committers have signed the CLA.