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.

global.go

@@ -1,11 +1,15 @@
 package kong
 
-import "os"
+import (
+	"os"
+)
 
 // Parse constructs a new parser and parses the default command-line.
 func Parse(cli interface{}, options ...Option) string {
 	parser, err := New(cli, options...)
-	parser.FatalIfErrorf(err)
+	if err != nil {
+		panic(err)
+	}
 	cmd, err := parser.Parse(os.Args[1:])
 	parser.FatalIfErrorf(err)
 	return cmd