add a button to remove optional values.

this does not clear the field, though.

clj/clarity/cljs/clarity.cljs

(defn make-typed-field [m optional?]
  (fn [[k t]]
    (dom/div #js {:className "field"}
      (when optional?
        (dom/input #js {:type "button", :value "-",
                        :onClick (fn [_] (om/update! m dissoc k))}))
      (dom/label nil (str k (when optional? " (optional)")))
      (om/build make-typed-input m {:opts {:type (om/value t), :key k, :val (k m)
                                           :optional? optional?}}))))