Add keygen command

cmd/keygen.go

+package cmd
+
+import (
+	"log"
+
+	"github.com/spf13/cobra"
+)
+
+var keygenCmd = &cobra.Command{
+	Use:   "keygen",
+	Short: "Generates a key(pair) for JWS signing and verifying.",
+	Long:  `Generates a key(pair) for JWS signing and verifying.`,
+	Run: func(cmd *cobra.Command, args []string) {
+		log.Fatalln("This command is not yet implemented!")
+	},
+}
+
+var use string
+var alg string
+
+func init() {
+	keygenCmd.Flags().StringVar(&alg, "alg", "", "Algorithm")
+	keygenCmd.Flags().StringVar(&use, "use", "", "Key usage (sign or enc)")
+	rootCmd.AddCommand(keygenCmd)
+}