[xttitle] Add option xttitle_short_format As suggested by Pavel Ulpi in issue #51: - A title is set as soon as the module is loaded, - The option xttitle_short_format can be set to 1 to have a very short title Notes: - The terminal title will only display the number of unread nuffers after the unread list has changed (not right after the module loading), - The title is reset to "MCabber" when the module is unloaded.