implement max-rank for pixiv

[thatfuckingbird]

I want to download the top N posts from a ranking page where N is configurable. I did not find a way to do this (image-range is not good since a pixiv post may have many images). This PR adds an option to achieve this.

[mikf]

The old problem of gallery-dl handling everything per-file instead of per-post ...

I think your solution can (and should) be applied more generally, so it can also be used for search results etc. I propose renaming this option to max-posts and using it for the main loop in items():

        works = self.works()
        if self.max_posts:
            works = itertools.islice(works, self.max_posts)
        for work in works:

[thatfuckingbird]

The old problem of gallery-dl handling everything per-file instead of per-post ...

I think your solution can (and should) be applied more generally, so it can also be used for search results etc. I propose renaming this option to max-posts and using it for the main loop in items():

        works = self.works()
        if self.max_posts:
            works = itertools.islice(works, self.max_posts)
        for work in works:

done!

[mikf]

Thank you!