Iterate over koans and add ns when init repl.

This pull request adds a way to iterate over all the koans in a koans project
much like in functional-koans/clojure-koans#68; except that it is
modularized and handled by the koan-engine.

It is an alternative to functional-koans/clojure-koans#71.

Other benefits are that it would be backwards compatible with any koans
that uses this engine and it does not need to be updated when new koans
are added.

The drawback is that it only solves the issue with vim-fireplace and not eclipse.