aryion: Support downloading specific folder

[kattjevfel]

Hi, sorry to open yet another issue about aryion (forgot to test this in the first issue). It seems it's currently possible to download either an entire artists gallery or single entries, but not a specific folder.

Example: https://aryion.com/g4/view/588928
A folder containing a comic, but I'm not interested in downloading the rest of their massive gallery.

Hope this isn't too much trouble to add in, I think I saw some "if not directory" checks or similar when looking at commits earlier.. :(