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_test.go

@@ -0,0 +1,31 @@
+package kong
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestParseHandlingBadBuild(t *testing.T) {
+	var cli struct {
+		Enabled bool `kong:"unknown"`
+	}
+
+	args := os.Args
+	defer func() {
+		os.Args = args
+	}()
+
+	os.Args = []string{os.Args[0], "hi"}
+
+	defer func() {
+		if r := recover(); r != nil {
+			require.Equal(t, Error{msg: "unknown is an unknown kong key"}, r)
+		}
+	}()
+
+	Parse(&cli, ExitFunction(func(_ int) { panic("exiting") }))
+
+	require.Fail(t, "we were expecting a panic")
+}