Allow to change default tags for Decoder and Encoder (#241)

Decoder: allow to customize default field name tag "toml" on decoding.
Example:
```
type doc struct {
    title `file:"header"`
}
```

Encoder: allow to customize tags for encoding struct to toml.
Example:
```
type doc struct {
    title `file:"header" description:"document title"`
}
```

Fixes #238

marshal.go

@@ -11,7 +11,12 @@ import (
 	"time"
 )
 
-const tagKeyMultiline = "multiline"
+const (
+	tagFieldName    = "toml"
+	tagFieldComment = "comment"
+	tagCommented    = "commented"
+	tagMultiline    = "multiline"
+)
 
 type tomlOpts struct {
 	name      string
@@ -31,6 +36,20 @@ var encOptsDefaults = encOpts{
 	quoteMapKeys: false,
 }
 
+type annotation struct {
+	tag       string
+	comment   string
+	commented string
+	multiline string
+}
+
+var annotationDefault = annotation{
+	tag:       tagFieldName,
+	comment:   tagFieldComment,
+	commented: tagCommented,
+	multiline: tagMultiline,
+}
+
 var timeType = reflect.TypeOf(time.Time{})
 var marshalerType = reflect.TypeOf(new(Marshaler)).Elem()
 
@@ -145,13 +164,15 @@ func Marshal(v interface{}) ([]byte, error) {
 type Encoder struct {
 	w io.Writer
 	encOpts
+	annotation
 }
 
 // NewEncoder returns a new encoder that writes to w.
 func NewEncoder(w io.Writer) *Encoder {
 	return &Encoder{
-		w:       w,
-		encOpts: encOptsDefaults,
+		w:          w,
+		encOpts:    encOptsDefaults,
+		annotation: annotationDefault,
 	}
 }
 
@@ -197,6 +218,30 @@ func (e *Encoder) ArraysWithOneElementPerLine(v bool) *Encoder {
 	return e
 }
 
+// SetTagName allows changing default tag "toml"
+func (e *Encoder) SetTagName(v string) *Encoder {
+	e.tag = v
+	return e
+}
+
+// SetTagComment allows changing default tag "comment"
+func (e *Encoder) SetTagComment(v string) *Encoder {
+	e.comment = v
+	return e
+}
+
+// SetTagCommented allows changing default tag "commented"
+func (e *Encoder) SetTagCommented(v string) *Encoder {
+	e.commented = v
+	return e
+}
+
+// SetTagMultiline allows changing default tag "multiline"
+func (e *Encoder) SetTagMultiline(v string) *Encoder {
+	e.multiline = v
+	return e
+}
+
 func (e *Encoder) marshal(v interface{}) ([]byte, error) {
 	mtype := reflect.TypeOf(v)
 	if mtype.Kind() != reflect.Struct {
@@ -227,7 +272,7 @@ func (e *Encoder) valueToTree(mtype reflect.Type, mval reflect.Value) (*Tree, er
 	case reflect.Struct:
 		for i := 0; i < mtype.NumField(); i++ {
 			mtypef, mvalf := mtype.Field(i), mval.Field(i)
-			opts := tomlOptions(mtypef)
+			opts := tomlOptions(mtypef, e.annotation)
 			if opts.include && (!opts.omitempty || !isZero(mvalf)) {
 				val, err := e.valueToToml(mtypef.Type, mvalf)
 				if err != nil {
@@ -326,7 +371,7 @@ func (e *Encoder) valueToToml(mtype reflect.Type, mval reflect.Value) (interface
 // Neither Unmarshaler interfaces nor UnmarshalTOML functions are supported for
 // sub-structs, and only definite types can be unmarshaled.
 func (t *Tree) Unmarshal(v interface{}) error {
-	d := Decoder{tval: t}
+	d := Decoder{tval: t, tagName: tagFieldName}
 	return d.unmarshal(v)
 }
 
@@ -362,6 +407,7 @@ type Decoder struct {
 	r    io.Reader
 	tval *Tree
 	encOpts
+	tagName string
 }
 
 // NewDecoder returns a new decoder that reads from r.
@@ -369,6 +415,7 @@ func NewDecoder(r io.Reader) *Decoder {
 	return &Decoder{
 		r:       r,
 		encOpts: encOptsDefaults,
+		tagName: tagFieldName,
 	}
 }
 
@@ -385,6 +432,12 @@ func (d *Decoder) Decode(v interface{}) error {
 	return d.unmarshal(v)
 }
 
+// SetTagName allows changing default tag "toml"
+func (d *Decoder) SetTagName(v string) *Decoder {
+	d.tagName = v
+	return d
+}
+
 func (d *Decoder) unmarshal(v interface{}) error {
 	mtype := reflect.TypeOf(v)
 	if mtype.Kind() != reflect.Ptr || mtype.Elem().Kind() != reflect.Struct {
@@ -410,7 +463,8 @@ func (d *Decoder) valueFromTree(mtype reflect.Type, tval *Tree) (reflect.Value,
 		mval = reflect.New(mtype).Elem()
 		for i := 0; i < mtype.NumField(); i++ {
 			mtypef := mtype.Field(i)
-			opts := tomlOptions(mtypef)
+			an := annotation{tag: d.tagName}
+			opts := tomlOptions(mtypef, an)
 			if opts.include {
 				baseKey := opts.name
 				keysToTry := []string{baseKey, strings.ToLower(baseKey), strings.ToTitle(baseKey)}
@@ -560,15 +614,15 @@ func (d *Decoder) unwrapPointer(mtype reflect.Type, tval interface{}) (reflect.V
 	return mval, nil
 }
 
-func tomlOptions(vf reflect.StructField) tomlOpts {
-	tag := vf.Tag.Get("toml")
+func tomlOptions(vf reflect.StructField, an annotation) tomlOpts {
+	tag := vf.Tag.Get(an.tag)
 	parse := strings.Split(tag, ",")
 	var comment string
-	if c := vf.Tag.Get("comment"); c != "" {
+	if c := vf.Tag.Get(an.comment); c != "" {
 		comment = c
 	}
-	commented, _ := strconv.ParseBool(vf.Tag.Get("commented"))
-	multiline, _ := strconv.ParseBool(vf.Tag.Get(tagKeyMultiline))
+	commented, _ := strconv.ParseBool(vf.Tag.Get(an.commented))
+	multiline, _ := strconv.ParseBool(vf.Tag.Get(an.multiline))
 	result := tomlOpts{name: vf.Name, comment: comment, commented: commented, multiline: multiline, include: true, omitempty: false}
 	if parse[0] != "" {
 		if parse[0] == "-" && len(parse) == 1 {