Add automatic metadata retrieval

[HDoujinDownloader]

This was partially implemented with metadata retrieval from the now-defunct doujinshi.org. It would be nice if the metadata lookups could be performed automatically.

Other discussions related to this issue:

• Metadata auto Scrapper?

[HDoujinDownloader]

A user suggested using doujinshi.wiki instead: https://doujindownloader.com/forum/viewtopic.php?f=12&t=2543