Kong tag PR fixes (#12)

* Removed bubbling errors in favour of panic
* Added a test for Parse with a bad build and an arg.
* Removed is() function in favour of a switch with strings
* Collect key and value in parseCSV, reducing complexity.
* Fix in global to not use parser instance on error.

kong.go

@@ -51,6 +51,7 @@ func New(ast interface{}, options ...Option) (*Kong, error) {
 	for _, option := range options {
 		option(k)
 	}
+
 	return k, nil
 }
 
@@ -75,11 +76,7 @@ func (k *Kong) Parse(args []string) (command string, err error) {
 }
 
 func (k *Kong) Errorf(format string, args ...interface{}) {
-	if k.Model != nil {
-		fmt.Fprintf(os.Stderr, k.Model.Name+": "+format, args...)
-	} else {
-		fmt.Fprintf(os.Stderr, format, args...)
-	}
+	fmt.Fprintf(os.Stderr, k.Model.Name+": "+format, args...)
 }
 
 func (k *Kong) FatalIfErrorf(err error, args ...interface{}) {