Added flags groups which are mutually exclusive but required

[lavafroth]

This commit adds flag groups which are mutually exclusive but required, as in, at least one of the flags in a group must be supplied.

For example:

rootCmd.Flags().StringVarP(&authToken, "auth-token", "t", "", "Authentication token (required if password is not set)")
rootCmd.Flags().StringVarP(&pw, "password", "p", "", "Password (required if authentication token is not set)")
rootCmd.MarkFlagsMutuallyExclusiveRequired("auth-token", "password")

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