Enable support for apps that are run locally at a sub-uri.

For example app_1 runs at dev.co/ and app_2 runs at dev.co/app_2/

If app_2 has BetterErrors installed, the repl will not show up because
it sends its `/__better_errors/...` requests to app_1.

This feature lets the user specify the sub-uri (or as its being called
here uri_prefix) in Rail's configuration block `config.better_error.uri_prefix = "/app_2"`
or by calling it directly `BetterErrors.uri_prefix = "/app_2"`.