Don't hardcode the version (#353)

and add a `version` command that only prints the version (without the
build time)

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

cmd/version.go

@@ -0,0 +1,19 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+func init() {
+	RootCmd.AddCommand(versionCmd)
+}
+
+var versionCmd = &cobra.Command{
+	Use:   "version",
+	Short: "Print the version of luet",
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Println(Version)
+	},
+}