[Request] Support unlisted illustrations

[daniels1989]

Apparently it's possible to create unlisted illustrations on Pixiv (I don't know how)

Examples: https://www.pixiv.net/en/artworks/unlisted/eE3fTYaROT9IsZmep386 https://www.pixiv.net/en/artworks/unlisted/nCjrDVZfPsbM9lmz5kge

Basically exact same url format as a normal image, just instead of a numeric id it's "unlisted/random 20 characters"

Ajax endpoint uses the same format: https://www.pixiv.net/ajax/illust/unlisted/eE3fTYaROT9IsZmep386 https://www.pixiv.net/ajax/illust/unlisted/nCjrDVZfPsbM9lmz5kge

Data returned is also the same