Preserve redirect with /_/ path prefix (#13)

This adds a straightforward way of preserving Farside's redirecting
behavior in the user's browser history. That way if an instance becomes
unavailable between the 5 min scans, the user can opt to navigate back
one page and be taken to a new instance.

This is accomplished using a single line of JS, and could potentially
work as the default behavior of Farside (with the current default
behavior requiring a path prefix instead). This should be revisited down
the road when more people are using this service.

lib/farside/router.ex

@@ -1,5 +1,6 @@
 defmodule Farside.Router do
   @index Application.fetch_env!(:farside, :index)
+  @route Application.fetch_env!(:farside, :route)
 
   use Plug.Router
 
@@ -24,6 +25,19 @@ defmodule Farside.Router do
     send_resp(conn, 200, resp)
   end
 
+  get "/_/:service/*glob" do
+    r_path = String.slice(conn.request_path, 2..-1)
+
+    resp =
+      EEx.eval_file(
+        @route,
+        service: service,
+        instance_url: r_path
+      )
+
+    send_resp(conn, 200, resp)
+  end
+
   get "/:service/*glob" do
     path = Enum.join(glob, "/")
     instance = Farside.pick_instance(service)