Scene skip

I allowed ren'py to ask a player to start fast skipping automatically when enter the seen label, that is like fate/stay night.

I added three actions:
         * Preference("scene skip", "enable") - enable scene_skip
         * Preference("scene skip", "disable") - disable scene_skip
         * Preference("scene skip", "toggle") - Toggle scene_skip

I added two config variable:
    config.default_scene_skip - if not None, the default value of scene_skip
    config.except_labels - the list of labels which should be ignored by scene_skip. 

scene_skip defaults to False
sample https://dl.dropboxusercontent.com/u/122050682/the_question-1.0-all.zip

I add labels which have "_" prefix or "_screen" suffix and following labels to ignore as special labels.
["enter_game_menu", "after_load", "splashscreen", "before_main_menu", "confirm_quit", "main_menu", "quit"]
Are there other special labels which should be ignored?