Add trailing dots to page.description when longer than 300 characters

lib/page/html.js

@@ -185,7 +185,11 @@ HTMLPipeline.prototype.applyAnnotations = function() {
 // This can totally be improved
 HTMLPipeline.prototype.extractDescription = function() {
     var $p = this.$('p');
-    var description = $p.text().trim().slice(0, 300);
+
+    var description = $p.text().trim();
+    if (description.length > 300) {
+        description = description.slice(0, 300).trim()+'...';
+    }
 
     this.opts.onDescription(description);
 };