ENH: filter for postprocessing function

[msurkovsky]

As there is a postprocess function, it is desireable to have a posibility to skip some slides from posprocessing. This commit adds such a functionality.

[spirali]

Thank you, but I am not going to merge this as it is an ad-hoc solution and not a systematic change. What if you want to do some post-processing for all slides but do not include some slides to page numbering?

Generally, this problem will be solved by tagging and naming, which is in my roadmap, but for now you can just directly use Python capabilities:

@elsie.slide()
def slide1(slide):
    slide.my_skip = True
    ...


def postprocessing(slides):
    slides = [s for s in slides if hasattr(s, "my_skip")]
    ....

elsie.render(slides_postprocessing=postprocessing)