Source::URL: add profile_url method.

Add a method for converting a source URL into a profile URL. This will
be used for normalizing profile URLs in artist entries.

Also add the ability to parse a few more profile URL formats.

app/logical/source/url/plurk.rb

@@ -38,4 +38,8 @@ class Source::URL::Plurk < Source::URL
   def image_url?
     host == "images.plurk.com"
   end
+
+  def profile_url
+    "https://www.plurk.com/#{username}" if username.present?
+  end
 end