Make {suburl} expand to {url}

This is useful for the following case from IRC:
`:set aliases '{"twmpv": "spawn mpv {suburl}"}'

which now sets:
:twmpv -> spawn mpv {url}

qutebrowser/commands/runners.py

@@ -65,6 +65,7 @@ def replace_variables(win_id, arglist):
             QUrl.DecodeReserved | QUrl.RemovePassword),
         'clipboard': utils.get_clipboard,
         'primary': lambda: utils.get_clipboard(selection=True),
+        'suburl': lambda: '{url}'
     }
     values = {}
     args = []