Add special handling for planck REPLs

[bbatsov]

Basically we need to extend what was done in #44 to cover planck as well, so all the inf-clojure functionality would work with planck properly.

This should be pretty easy and I would have done it myself if I knew the first thing about planck. :-)

[slipset]

Lucky you, it's my fun-Friday tomorrow, so I guess this will be the task.

[arichiardi]

I want to duplicate the load form defcustom today so that you have it tomorrow for planck ;)