Add HTTP proxy support.

Add support for using a proxy for HTTP requests. Only used for external
requests, such as downloading files or talking to source sites such as
Pixiv or Twitter, not for internal requests, such as talking to IQDB or
Reportbooru.

config/danbooru_default_config.rb

@@ -494,6 +494,18 @@ module Danbooru
       false
     end
 
+    def http_proxy_host
+    end
+
+    def http_proxy_port
+    end
+
+    def http_proxy_username
+    end
+
+    def http_proxy_password
+    end
+
     # The URL for the Reportbooru server (https://github.com/evazion/reportbooru).
     # Optional. Used for tracking post views, popular searches, and missed searches.
     # Set to http://localhost/mock/reportbooru to enable a fake reportbooru