Use unique key for html collection elements

[tristanstraub]

• Added function html-val which uses an index to set the key
• Modified join-html to call html-val with an index instead of html

This fixes the following card which complains about duplicate keys in react:

(defcard devcard-inspect
  (fn [data _]
    (reagent/as-element
     [:button {:on-click #(swap! data conj ".")} "Just one bite!"]))
  ["."]
  {:inspect-data true})