Philomena booru support.

[SilkRose]

Could you add support for downloading from other boorus that have the same API as derpibooru? https://ponybooru.org/ for example. The API is exactly the same and the only differences would be the URL and the Everything filter ID probably.

[mikf]

Added support for https://ponybooru.org/ in c485d0a

To get gallery-dl to recogize other Philomena sites, do the same as in the description of 595bdaa, but use "philomena" instead of "gelbooru_v01".