cmd/ghcs/main: Add --version flag

- This is built into Cobra the argument parser. Now `ghcs --version`
  exists.
- When we prepare to bump the version, we need to remember to update
  this value else the Homebrew formula, GitHub releases and the `ghcs
  --version` output will be mismatched.
- Fixes https://github.com/github/ghcs/issues/16.

cmd/ghcs/main.go

@@ -16,9 +16,10 @@ func main() {
 }
 
 var rootCmd = &cobra.Command{
-	Use:   "ghcs",
-	Short: "Codespaces",
-	Long:  "Codespaces",
+	Use:     "ghcs",
+	Short:   "Codespaces",
+	Long:    "Codespaces",
+	Version: "0.5.0",
 }
 
 func Execute() {