Use "kong" as tag keys fixes #9 (#10)

global.go

@@ -3,9 +3,10 @@ package kong
 import "os"
 
 // Parse constructs a new parser and parses the default command-line.
-func Parse(cli interface{}, options ...Option) {
+func Parse(cli interface{}, options ...Option) string {
 	parser, err := New(cli, options...)
 	parser.FatalIfErrorf(err)
-	_, err = parser.Parse(os.Args[1:])
+	cmd, err := parser.Parse(os.Args[1:])
 	parser.FatalIfErrorf(err)
+	return cmd
 }