Add Sinatra::Subroute, allowing an app to make a request to another endpoint in the app.

Allows for, e.g.:

```ruby
class App < Sinatra::Application
  helpers Sinatra::Subroute
  get '/target' do
    ...
  end

  get '/subrouted' do
    subroute!('/target')
  end
end
```

subroute! can also add or modify request params and the HTTP verb used.

(Originally from https://github.com/lookout/lookout-rack-utils)