Add configuration loading + docs + linter fixes.

options_test.go

@@ -1,6 +1,9 @@
 package kong
 
 import (
+	"encoding/json"
+	"io/ioutil"
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -16,3 +19,31 @@ func TestOptions(t *testing.T) {
 	require.Nil(t, p.Stderr)
 	require.Nil(t, p.Exit)
 }
+
+func TestConfigLoading(t *testing.T) {
+	first, err := ioutil.TempFile("", "")
+	require.NoError(t, err)
+	defer first.Close()
+	defer os.Remove(first.Name())
+	second, err := ioutil.TempFile("", "")
+	require.NoError(t, err)
+	defer second.Close()
+	defer os.Remove(second.Name())
+
+	var cli struct {
+		Flag string `json:"flag,omitempty"`
+	}
+
+	cli.Flag = "first"
+	err = json.NewEncoder(first).Encode(&cli)
+	require.NoError(t, err)
+
+	cli.Flag = ""
+	err = json.NewEncoder(second).Encode(&cli)
+	require.NoError(t, err)
+
+	p := mustNew(t, &cli, Configuration(JSON, first.Name(), second.Name()))
+	_, err = p.Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "first", cli.Flag)
+}