Add ability to record Rails request ID

~~**Merge after #37 - I branched off that branch to avoid creating conflicts with my own PR**~~ Done

Something I find really useful in Rails is the ability to correlate cross-service requests [using consistent request IDs](http://api.rubyonrails.org/classes/ActionDispatch/RequestId.html). I'd love to have these recorded in query logs as well, so I can correlate DB calls all the way back up to a request hitting the web server at the front.

This PR adds that ability to Marginalia.

cc @arthurnn 
That's there to handle cases where `@controller` isn't set. The code in the methods above seems to use a mixture of checking `@controller.respond_to?` and `if @controller`. Not sure which style is preferred.