Also fixes the uploader uploading all images when trying to upload only a
single image in a multi-image work. Caused by `image_urls` incorrectly
returning all images when the source strategy was given a url for a
single image.
Add `#basename`, `#filename`, and `#file_ext` utility methods to
Danbooru::URL and change a few places to use them. Simplifies parsing
filenames in source URLs in various places.
Introduce a Source::URL class for parsing URLs from source sites. Refactor the Twitter
source strategy to use it.
This is the first step towards factoring all the URL parsing logic out of source
strategies and moving it to subclasses of Source::URL. Each site will have a subclass
of Source::URL dedicated to parsing URLs from that site. Source strategies will use
these classes to extract information from URLs.
This is to simplify source strategies. Most sites have many different URL formats we have
to parse or rewrite, and handling all these different cases tends to make source
strategies very complex. Isolating the URL parsing logic from the site scraping logic
should make source strategies easier to maintain.
