Note: Allow extra yaml metadata to be set

This way if there is existing metadata, we aren't removing it when
writing the notes. The order isn't preserved, but meh, that's fine for
now.

lib/note.dart

@@ -9,12 +9,15 @@ class Note implements Comparable {
   DateTime created;
   String body;
 
-  Note({this.created, this.body, this.id});
+  Map<String, dynamic> extraProperties = new Map<String, dynamic>();
+
+  Note({this.created, this.body, this.id, this.extraProperties});
 
   factory Note.fromJson(Map<String, dynamic> json) {
     String id;
     if (json.containsKey("id")) {
       id = json["id"].toString();
+      json.remove("id");
     }
 
     DateTime created;
@@ -36,8 +39,11 @@ class Note implements Comparable {
 
       // FIXME: Get created from file system or from git!
       if (created == null) {
+        // FIXME: make this 0
         created = DateTime.now();
       }
+
+      json.remove("created");
     }
 
     if (id == null && created != null) {
@@ -47,25 +53,29 @@ class Note implements Comparable {
     String body = "";
     if (json.containsKey("body")) {
       body = json['body'];
+      json.remove("body");
     }
 
     return new Note(
       id: id,
       created: created,
       body: body,
+      extraProperties: json,
     );
   }
 
   Map<String, dynamic> toJson() {
-    return {
-      "created": toIso8601WithTimezone(created),
-      "body": body,
-      "id": id,
-    };
+    var json = Map<String, dynamic>.from(extraProperties);
+    json['created'] = toIso8601WithTimezone(created);
+    json['body'] = body;
+    json['id'] = id;
+
+    return json;
   }
 
   @override
-  int get hashCode => id.hashCode ^ created.hashCode ^ body.hashCode;
+  int get hashCode =>
+      id.hashCode ^ created.hashCode ^ body.hashCode ^ extraProperties.hashCode;
 
   @override
   bool operator ==(Object other) =>
@@ -74,11 +84,12 @@ class Note implements Comparable {
           runtimeType == other.runtimeType &&
           id == other.id &&
           body == other.body &&
-          created == other.created;
+          created == other.created &&
+          extraProperties == other.extraProperties;
 
   @override
   String toString() {
-    return 'Note{id: $id, body: $body, created: $created}';
+    return 'Note{id: $id, body: $body, created: $created, extraProperties: $extraProperties}';
   }
 
   @override