Check out Light Table. It has a really interesting architecture specifically designed to support that kind of interactive scriptability / extensibility, and the

Check out Light Table. It has a really interesting architecture specifically designed to support that kind of interactive scriptability / extensibility, and the fact that it's written in a decent Lisp (Clojure) is the icing on the cake. It's a joy to work with, because even when the internal code has problems you can just patch it on the fly.