description & link as non mandatory fields

lib/domain/rss_feed.dart

@@ -53,8 +53,8 @@ class RssFeed {
       throw new ArgumentError("channel not found");
     }
     var title = xmlGetString(channelElement, "title");
-    var description = xmlGetString(channelElement, "description");
-    var link = xmlGetString(channelElement, "link");
+    var description = xmlGetString(channelElement, "description", strict: false);
+    var link = xmlGetString(channelElement, "link", strict: false);
 
     var feeds = channelElement.findElements("item").map((element) {
       return new RssItem.parse(element);

lib/domain/rss_item.dart

@@ -32,8 +32,8 @@ class RssItem {
 
   factory RssItem.parse(XmlElement element) {
     var title = xmlGetString(element, "title");
-    var description = xmlGetString(element, "description");
-    var link = xmlGetString(element, "link");
+    var description = xmlGetString(element, "description", strict: false);
+    var link = xmlGetString(element, "link", strict: false);
 
     var categories = element.findElements("category").map((element) {
       return new RssCategory.parse(element);