Move default viewers to vars

[mk]

To make inspecting & building on them easier. Use this to implement an example dashboard viewer that only shows the last result. This can be useful to build apps or dashboard without needing to hide every cell & result except one.

(def last-result-viewer
  (update v/notebook-viewer :transform-fn (fn [transform-fn-orig]
                                            (comp transform-fn-orig
                                                  (clerk/update-val (fn [doc] (update doc :blocks (partial take-last 1))))))))

Fixes #133.