Make the add-middleware function public

[jcrossley3]

I'd love for immutant.web/start to accept the same handy dev-mode
options provided by ring-server, but I'd rather not rewrite the code.
If this function was exposed, I'd be set! :) 

[jcrossley3]

I tried using the function as is by referring to its var, only to realize that standalone pulls in ring.adapter.jetty that I really don't need. I'd almost prefer those add-* functions moved to the options ns, or possibly a new one entirely. I'd hate to pull in the jetty dep, but I'd like to re-use the logic for adding the middleware. Are you open to that?