Fix spawn --userscript with multiple args.

This is a regression introduced in 4485e4ee1b.

We didn't unpack the argument list properly before passing it to
run_userscripts.

Fixes #476.
See #448.

qutebrowser/browser/commands.py

@@ -813,10 +813,9 @@ class CommandDispatcher:
         log.procs.debug("Executing: {}, userscript={}".format(
             args, userscript))
         if userscript:
-            if len(args) > 1:
+            cmd = args[0]
-                self.run_userscript(args[0], args[1:])
+            args = [] if not args else args[1:]
-            else:
+            self.run_userscript(cmd, *args)
-                self.run_userscript(args[0])
         else:
             try:
                 subprocess.Popen(args)