Refactor how output flag works

This changes the -o or --output flag so that it determines where the
Shortcut will output and what it will be named.

main.go

@@ -28,7 +28,7 @@ func main() {
 	registerArg("share", "s", "Signing mode. [anyone, contacts] [default=contacts]")
 	registerArg("unsigned", "u", "Don't sign compiled Shortcut. Will NOT run on iOS or macOS.")
 	registerArg("debug", "d", "Save generated plist. Print debug messages and stack traces.")
-	registerArg("output", "o", "Path to save the shortcut. (e.g. path/to/)")
+	registerArg("output", "o", "Optional output file path. (e.g. /path/to/file.shortcut).")
 	if len(os.Args) < 2 {
 		usage()
 		os.Exit(0)
@@ -156,7 +156,7 @@ func sign() {
 	}
 	var outputPath = basename + ".shortcut"
 	if arg("output") {
-		outputPath = argValue("output") + outputPath
+		outputPath = argValue("output")
 	}
 	var signBytes, signErr = exec.Command(
 		"shortcuts",