Danbooru::URL: add #basename, #filename, and #file_ext utility methods.

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.

app/logical/source/url/plurk.rb

@@ -40,7 +40,7 @@ class Source::URL::Plurk < Source::URL
     # https://images.plurk.com/5wj6WD0r6y4rLN0DL3sqag.jpg
     # https://images.plurk.com/mx_5wj6WD0r6y4rLN0DL3sqag.jpg
     in "plurk.com", /^(mx_)?(\w{22})\.(\w+)$/
-      @filename, @file_ext = $2, $3
+      @image_id = $2
 
     # https://www.plurk.com/p/om6zv4
     in "plurk.com", "p", work_id