Strip the url before processing it

This won't hurt and will help with some poorly formatted sites
including blank spaces around the url (e.g. the Previous link in a
dashboard make with CDash 2.0.2).

qutebrowser/browser/hints.py

@@ -604,7 +604,7 @@ class HintManager(QObject):
         """
         for attr in ('href', 'src'):
             if attr in elem:
-                text = elem[attr]
+                text = elem[attr].strip()
                 break
         else:
             return None