[realbooru] Add Realbooru extractor

[TheOddball]

Realbooru uses the same version of booru as gel, so I used that extractor with changed variables, works perfectly.

[mikf]

Instead of copy-pasting and adjusting the entire Gelbooru code, you should do this with the code from safebooru.py (or rule34.py). Much less boilerplate.
(I'm also not the copyright holder for realbooru.py, by the way)

As for tests: Remove all the test result sections, so that everything looks like

    test = ("https://realbooru.com/…",)

and actually passes the CI. Or run python test/test_results realbooru locally and adjust the test results until it passes.

I should also mention that there will, at some point, be general extractors for everything …booru related (#234, #473) and this, among all other idividual …booru extractors, is most likely going to be deleted at that point.

[TheOddball]

Well.. To be fair I'm not a Python programmer. I merely edited the gelbooru one to work, feel free to close this if it's not good enough

[mikf]

It is good enough, don't worry about it.

Instead of editing the Gelbooru file, you could have done so with the file for Safebooru and it would have been at lot less code. I've done that now myself and also "fixed" the tests I was talking about: 31a2983.

Thank you for your contribution.