Implemented most of the base parser.

This includes branching arguments as well as commands, eg.

    app user create <id> <first> <last>
    app user <id> delete
    app user <id> rename <to>

Of note, required/optional flags and positional arguments are not
currently enforced.

kong_test.go

@@ -0,0 +1,63 @@
+package kong
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/alecthomas/repr"
+)
+
+func mustNew(t *testing.T, cli interface{}) *Kong {
+	t.Helper()
+	parser, err := New("", "", cli)
+	require.NoError(t, err)
+	return parser
+}
+
+func TestArgument(t *testing.T) {
+	/*
+		app user create <id> <first> <last>
+		app	user <id> delete
+		app	user <id> rename <to>
+
+	*/
+	var cli struct {
+		Create struct {
+			Id    string `arg:"true"`
+			First string `arg:"true"`
+			Last  string `arg:"true"`
+		}
+
+		// Branching argument.
+		Id struct {
+			Id     int `arg:"true"`
+			Flag   int
+			Delete struct{}
+			Rename struct {
+				To string
+			}
+		} `arg:"true"`
+	}
+	p := mustNew(t, &cli)
+	repr.Println(p.Model, repr.Hide(reflect.Value{}))
+	cmd, err := p.Parse([]string{"10", "delete"})
+	require.NoError(t, err)
+	require.Equal(t, 10, cli.Id.Id)
+	require.Equal(t, "<id> delete", cmd)
+}
+
+func TestResetWithDefaults(t *testing.T) {
+	var cli struct {
+		Flag            string
+		FlagWithDefault string `default:"default"`
+	}
+	cli.Flag = "BLAH"
+	cli.FlagWithDefault = "BLAH"
+	parser := mustNew(t, &cli)
+	_, err := parser.Parse([]string{})
+	require.NoError(t, err)
+	require.Equal(t, "", cli.Flag)
+	require.Equal(t, "default", cli.FlagWithDefault)
+}