redirect to originally requested page upon login

One way (suggested by @essenciary [here](https://gitter.im/essenciary/Genie.jl)) would be to include the original URL as an optional param like
`redirect("login?original_page=/users/10/delete")`
or
`redirect(:show_login; extra_query = Dict("original_page" => "/users/10/delete")`

I also wondered if it could be stored in the session object? I'm not really a web designer so maybe this is stupid for some reason, I don't know.

I'm just not sure how to get the last requested URL.