Add meta-viewer for metadata-based viewer assignment

[sritchie]

@mk and I came up with this approach working on emmy-viewers together.

Another viewer I'd like to suggest for inclusion is a "reagent-viewer"... or maybe a render-fn-viewer. It acts on quoted reagent forms.

The existing reagent-viewer errors for reagent forms like

(clerk/with-viewer nextjournal.clerk.viewer/reagent-viewer
  '(reagent.core/with-let [!click (reagent.core/atom 0)]
     [:div
      "The atom " [:code "!click"] " has value: "
      @!click ". "
      [:input
       {:type :button
        :value "Click me!"
        :on-click
        (fn [e]
          (js/console.log e)
          (.stopPropagation e)
          (swap! !click inc))}]]))

Here is the viewer:

(defn- strip-meta [form]
  (postwalk (fn [x]
              (if (meta x)
                (vary-meta x dissoc ::clerk/viewer)
                x))
            form))

(def reagent-viewer
  {:name `reagent-viewer
   :transform-fn
   (clerk/update-val
    (fn [form]
      (clerk/with-viewer
        {:render-fn
         (list 'fn [] (strip-meta form))}
        nil)))})

And here is it working with that reagent form above:

[sritchie]

I am happy to add examples to the book, @mk .

Here's one: