An init similar to Rails.init but for Rack apps (i.e. Rack.init)

I like using configatron with rails, but more often these days I don't have a rails app but a rack app instead (e..g sinatra), however I still like to use the same configuration file structure. So, just a quick patch to add Configatron::Rack.init. Works exactly the same as its Rails counterpart if you call from config.ru for example.