Add selenium as optional dependency

[thatDudo]

Once cloudflare or some javascript obfuscation comes into play it becomes less reasonable to not let some kind of browser deal with it. Tachiyomi is already doing that as you surely know.
Would you consider using selenium or accepting PR's regarding that or do you think that it doesnt belong into gallery-dl?
It could be made optional too, so that if it isnt installed the current methods could be used as a fallback.

Extractors who would profit from this are for example: ,

[mikf]

I think selenium does not belong into gallery-dl.

Extractors who would profit from this are for example: nhentai, pixiv

Retrieving the necessary cookies or tokens can be automated outside of gallery-dl. All you need to do is pass those to gallery-dl to make it work.