added "suspend_rollback()" functionality

Allows checkpoint saving to be suspended, which means rolling back will skip over the current code to prior to the suspension.  Useful for minigames, where you'd rather people not be able to back up into different states, but you don't want to block rollback entirely.

Example code:

    label start:

        scene black
        "test of rollback skipping"

        "should rollback to here"

        $ renpy.suspend_rollback(True)

        "you won't see this line in rollback"

        "or this one"

        $ renpy.suspend_rollback(False)

        "roll back now"

        return