Support aliases for flags (#409)

Aliases are currently only supported for sub-commands, but they're
useful for flags as well. E.g., when migrating from an old flag name
to a new flag name, while still supporting the old value.

kong_test.go

@@ -518,6 +518,16 @@ func TestShort(t *testing.T) {
 	assert.Equal(t, "hello", cli.String)
 }
 
+func TestAlias(t *testing.T) {
+	var cli struct {
+		String string `aliases:"str"`
+	}
+	app := mustNew(t, &cli)
+	_, err := app.Parse([]string{"--str", "hello"})
+	assert.NoError(t, err)
+	assert.Equal(t, "hello", cli.String)
+}
+
 func TestDuplicateFlagChoosesLast(t *testing.T) {
 	var cli struct {
 		Flag int
@@ -1321,6 +1331,24 @@ func TestDuplicateShortflags(t *testing.T) {
 	assert.EqualError(t, err, "<anonymous struct>.Flag2: duplicate short flag -t")
 }
 
+func TestDuplicateAliases(t *testing.T) {
+	cli1 := struct {
+		Flag1 string `aliases:"flag"`
+		Flag2 string `aliases:"flag"`
+	}{}
+	_, err := kong.New(&cli1)
+	assert.EqualError(t, err, "<anonymous struct>.Flag2: duplicate flag --flag")
+}
+
+func TestDuplicateAliasLong(t *testing.T) {
+	cli2 := struct {
+		Flag  string ``
+		Flag2 string `aliases:"flag"` // duplicates Flag
+	}{}
+	_, err := kong.New(&cli2)
+	assert.EqualError(t, err, "<anonymous struct>.Flag2: duplicate flag --flag")
+}
+
 func TestDuplicateNestedShortFlags(t *testing.T) {
 	cli := struct {
 		Flag1 bool `short:"t"`