Refactored decoder responsibilities (#1402)

- improved comment handling
- yaml decoder now responsible for leading content work around

pkg/yqlib/decoder_json.go

@@ -16,26 +16,31 @@ func NewJSONDecoder() Decoder {
 	return &jsonDecoder{}
 }
 
-func (dec *jsonDecoder) Init(reader io.Reader) {
+func (dec *jsonDecoder) Init(reader io.Reader) error {
 	dec.decoder = *json.NewDecoder(reader)
+	return nil
 }
 
-func (dec *jsonDecoder) Decode(rootYamlNode *yaml.Node) error {
+func (dec *jsonDecoder) Decode() (*CandidateNode, error) {
 
 	var dataBucket orderedMap
 	log.Debug("going to decode")
 	err := dec.decoder.Decode(&dataBucket)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	node, err := dec.convertToYamlNode(&dataBucket)
 
 	if err != nil {
-		return err
+		return nil, err
 	}
-	rootYamlNode.Kind = yaml.DocumentNode
-	rootYamlNode.Content = []*yaml.Node{node}
-	return nil
+
+	return &CandidateNode{
+		Node: &yaml.Node{
+			Kind:    yaml.DocumentNode,
+			Content: []*yaml.Node{node},
+		},
+	}, nil
 }
 
 func (dec *jsonDecoder) convertToYamlNode(data *orderedMap) (*yaml.Node, error) {