Add `:wrap` option for adding middleware

[edw]

I've found myself with the occasional need to expose a Gorilla REPL session to the outside world, and I've done so by adding HTTP basic authentication middleware. There's currently no way to do this without modifying code. Thus this pull request.

One unpleasant side effect of the approach taken here is that the server app isn't bound to a var, and therefore can't be var-quoted to ease testing.