automatically define composite images

I allowed Ren'Py to also define composite images automatically.

I added two config variables

.. var:: config.automatic_images_parts = None

    If this and `config.automatic_images` are not None, this causes Ren'Py
    to automatically also define composite images.

    When not set to None, this should be set to a list of strings giving
    directories that include images which are parts of composite images.

    Parts are composited from left to right and all parts must be same size.

.. var:: config.automatic_images_not_necessary_parts = []

    A list of strings giving directories including images which are not
    necessary parts for composite images.


    For example, if relative variables are set like below:

    * config.automatic_images = ["/"]
    * config.automatic_images_parts = ["body", "face", "sweat", "surplus"]
    * config.automatic_images_not_necessary_parts = ["sweat"]

    and your game directly contains, :

    * eileen/body/b1.png
    * eileen/body/b2.png
    * eileen/face/f1.png
    * eileen/face/f2.png
    * eileen/sweat/sweat.png

    then below images are defined :

    * eileen b1 f1
    * eileen b1 f1 sweat 
    * eileen b1 f2
    * eileen b1 f2 sweat 
    * eileen b2 f1
    * eileen b2 f1 sweat 
    * eileen b2 f2
    * eileen b2 f2 sweat