[Tmohentai] add support

[JSouthGB]

For site-request #4808. You can pass the main content URL or any individual page URL. No support for groups or tags as of right now (I can work on adding them if someone will find it useful).

I'm open to any feedback, particularly the metadata retrieval.

[mikf]

You should inherit from GalleryExtractor and use its interface, like for example in architizer.py or nhentai.py.

As for metadata retrieval, text.extr() "works", but it would be better to use text.extract_from(page) and use it to extract data in order of appearance in the site's HTML since it stores the current position internally.