Added a --toggle switch to "mark [--toggle] <mark>"

This option allows toggling marks on a window without knowing whether the mark is already set or not.
It behaves as follows:
1) If the matched window has no mark, the new mark is set.
2) If the matched window has another mark, the old mark is removed and the new mark is set.
3) If the matched window already has the mark, the mark is removed.

The behavior that all non-matched windows have this mark removed is kept.

fixes #1463

parser-specs/commands.spec

@@ -189,10 +189,12 @@ state FLOATING:
   floating = 'enable', 'disable', 'toggle'
       -> call cmd_floating($floating)
 
-# mark <mark>
+# mark [--toggle] <mark>
 state MARK:
+  toggle = '--toggle'
+      ->
   mark = string
-      -> call cmd_mark($mark)
+      -> call cmd_mark($mark, $toggle)
 
 # unmark [mark]
 state UNMARK: