Extract remote ip from X-Forwarded-For

The remote IP returned in plug.conn is meant to be overwritten by the
developer, and doesn't reflect the origin of the request based on
headers. A new dependency has been added to make this change quicker to
roll out.

mix.exs

@@ -42,6 +42,7 @@ defmodule Farside.MixProject do
       {:plug_attack, "~> 0.4.2"},
       {:plug_cowboy, "~> 2.0"},
       {:quantum, "~> 3.0"},
+      {:remote_ip, "~> 1.1"},
       {:cubdb, "~> 2.0.1"},
       {:bakeware, "~> 0.2.4"}
     ]