Support sending nREPL a custom rendering function

Hi, over the past few days I've been hacking together [Puget](https://github.com/greglook/puget) and the Leiningen REPL to try to get pretty-printing enabled by default for REPL output. The result was [whidbey](https://github.com/greglook/whidbey), an nREPL middleware.

In the course of this, I needed to make a small change to REPLy to pass along some configuration to nREPL. What do you think of my implementation? I think I've got a grasp on how REPLy and nREPL interact, but if you have suggestions for a better way to do this I'd love to hear it.