Implement flag "resolvers". (#24)

* Propagate errors.
* Use junit test output.
* Expand role of DecodeContext to include Scanner.
* Inject resolved flags as Path elements in the Context.
  This allows all existing logic to apply seamlessly: hooks, required
flags, etc.
* Clarify that hooks can be called multiple times.

kong_test.go

@@ -101,9 +101,19 @@ func TestFlagSlice(t *testing.T) {
 	require.Equal(t, []int{1, 2, 3}, cli.Slice)
 }
 
+func TestFlagSliceWithSeparator(t *testing.T) {
+	var cli struct {
+		Slice []string
+	}
+	parser := mustNew(t, &cli)
+	_, err := parser.Parse([]string{`--slice=a\,b,c`})
+	require.NoError(t, err)
+	require.Equal(t, []string{"a,b", "c"}, cli.Slice)
+}
+
 func TestArgSlice(t *testing.T) {
 	var cli struct {
-		Slice []int `kong:"arg"`
+		Slice []int `arg`
 		Flag  bool
 	}
 	parser := mustNew(t, &cli)
@@ -113,6 +123,18 @@ func TestArgSlice(t *testing.T) {
 	require.Equal(t, true, cli.Flag)
 }
 
+func TestArgSliceWithSeparator(t *testing.T) {
+	var cli struct {
+		Slice []string `arg`
+		Flag  bool
+	}
+	parser := mustNew(t, &cli)
+	_, err := parser.Parse([]string{"a,b", "c", "--flag"})
+	require.NoError(t, err)
+	require.Equal(t, []string{"a,b", "c"}, cli.Slice)
+	require.Equal(t, true, cli.Flag)
+}
+
 func TestUnsupportedFieldErrors(t *testing.T) {
 	var cli struct {
 		Keys map[string]string
@@ -356,3 +378,23 @@ func TestShort(t *testing.T) {
 	require.True(t, cli.Bool)
 	require.Equal(t, "hello", cli.String)
 }
+
+func TestDuplicateFlagChoosesLast(t *testing.T) {
+	var cli struct {
+		Flag int
+	}
+
+	_, err := mustNew(t, &cli).Parse([]string{"--flag=1", "--flag=2"})
+	require.NoError(t, err)
+	require.Equal(t, 2, cli.Flag)
+}
+
+func TestDuplicateSliceDoesNotAccumulate(t *testing.T) {
+	var cli struct {
+		Flag []int
+	}
+
+	_, err := mustNew(t, &cli).Parse([]string{"--flag=1,2", "--flag=3,4"})
+	require.NoError(t, err)
+	require.Equal(t, []int{3, 4}, cli.Flag)
+}