Experimental feature: named sessions in uzbl-tabbed

As in the previous pull request, named sessions are files in config['saved_sessions_dir'], and there are bindings in the example config file.
In the command-line interface, flag '-l <name>' loads (opens) a new tabbed instance with the saved tabs in this session and '-L' lists the existing named sessions.
In uzbl-tabbed, dmenu is used to list and open existing sessions, while an actual argument is necessary for 'delete', 'save' (create a new named session from the current tabs in the browser) and 'merge' (which.. well, merges the current session with the one in the argument).