Option for a custom pretty printer

I wrote a [new pretty printer](https://github.com/brandonbloom/fipp) that I would love to install into `~/.lein/profile.clj`. Unfortunately, REPL-y's default initialization code seems to be all-or-nothing.

The attached patch does the brain dead bare minimum (afaict) to add a `:require-pprint` option. My goal is to make this work:

```clojure
{:user {:repl-options {:require-pprint [bbloom.fipp.edn :refer (pprint)]}
        :dependencies [[fipp "0.1.0-SNAPSHOT"]]}}
```

I wrote this patch with blinders on to the rest of the REPLy codebase, so feel free to suggest a different approach.

On a slightly more ambitious note: Fipp might be fast enough to be used as a default printer in nREPL. I figure that I'll propose that change to Cemerick once I'm confident that I have a true drop-in replacement for `clojure.pprint`. I need to be using Fipp by default if there is any hope of it maturing to that point.