marshal: do not encode embedded structs as sub-table (#368)

Currently, the marshalling code encodes the embedded structs as sub-tables.
This is a bit unexpected, as it differs from what encoding/json does in
that case: https://play.golang.org/p/KDPaGtrijV1

Unmarshalling code handles this scenario gracefully.

This PR adapts the encoder to behave like encoding/json.
Fields in an embedded struct are promoted to the top level table.
In case the embedded struct is named in the tag, it will still
encode as a sub-table.

The added PromoteAnonymous option on the Encoder allows configuring
the old behavior, where anonymous structs are encoded as sub-tables.

On duplicate keys, the behavior of encoding/json is mimicked:
Fields from anonymous structs are shadowed by regular fields.

An example is added to show the affects of setting PromoteAnonymous.

marshal.go

@@ -22,6 +22,7 @@ const (
 
 type tomlOpts struct {
 	name         string
+	nameFromTag  bool
 	comment      string
 	commented    bool
 	multiline    bool
@@ -190,9 +191,10 @@ type Encoder struct {
 	w io.Writer
 	encOpts
 	annotation
-	line  int
-	col   int
-	order marshalOrder
+	line        int
+	col         int
+	order       marshalOrder
+	promoteAnon bool
 }
 
 // NewEncoder returns a new encoder that writes to w.
@@ -279,6 +281,19 @@ func (e *Encoder) SetTagMultiline(v string) *Encoder {
 	return e
 }
 
+// PromoteAnonymous allows to change how anonymous struct fields are marshaled.
+// Usually, they are marshaled as if the inner exported fields were fields in
+// the outer struct. However, if an anonymous struct field is given a name in
+// its TOML tag, it is treated like a regular struct field with that name.
+// rather than being anonymous.
+//
+// In case anonymous promotion is enabled, all anonymous structs are promoted
+// and treated like regular struct fields.
+func (e *Encoder) PromoteAnonymous(promote bool) *Encoder {
+	e.promoteAnon = promote
+	return e
+}
+
 func (e *Encoder) marshal(v interface{}) ([]byte, error) {
 	mtype := reflect.TypeOf(v)
 	if mtype == nil {
@@ -338,12 +353,15 @@ func (e *Encoder) valueToTree(mtype reflect.Type, mval reflect.Value) (*Tree, er
 					if err != nil {
 						return nil, err
 					}
-
-					tval.SetWithOptions(opts.name, SetOptions{
-						Comment:   opts.comment,
-						Commented: opts.commented,
-						Multiline: opts.multiline,
-					}, val)
+					if tree, ok := val.(*Tree); ok && mtypef.Anonymous && !opts.nameFromTag && !e.promoteAnon {
+						e.appendTree(tval, tree)
+					} else {
+						tval.SetWithOptions(opts.name, SetOptions{
+							Comment:   opts.comment,
+							Commented: opts.commented,
+							Multiline: opts.multiline,
+						}, val)
+					}
 				}
 			}
 		}
@@ -460,6 +478,19 @@ func (e *Encoder) valueToToml(mtype reflect.Type, mval reflect.Value) (interface
 	}
 }
 
+func (e *Encoder) appendTree(t, o *Tree) error {
+	for key, value := range o.values {
+		if _, ok := t.values[key]; ok {
+			continue
+		}
+		if tomlValue, ok := value.(*tomlValue); ok {
+			tomlValue.position.Col = t.position.Col
+		}
+		t.values[key] = value
+	}
+	return nil
+}
+
 // Unmarshal attempts to unmarshal the Tree into a Go struct pointed by v.
 // Neither Unmarshaler interfaces nor UnmarshalTOML functions are supported for
 // sub-structs, and only definite types can be unmarshaled.
@@ -913,6 +944,7 @@ func tomlOptions(vf reflect.StructField, an annotation) tomlOpts {
 	defaultValue := vf.Tag.Get(tagDefault)
 	result := tomlOpts{
 		name:         vf.Name,
+		nameFromTag:  false,
 		comment:      comment,
 		commented:    commented,
 		multiline:    multiline,
@@ -925,6 +957,7 @@ func tomlOptions(vf reflect.StructField, an annotation) tomlOpts {
 			result.include = false
 		} else {
 			result.name = strings.Trim(parse[0], " ")
+			result.nameFromTag = true
 		}
 	}
 	if vf.PkgPath != "" {