Allows for an arbitrary number of props in merge-props

[jimberlage]

We've run into a few instances of people trying to use reagent.core/merge-props like clojure.core/merge, with multiple maps. For an example, think of something like this:

(r/merge-props
  {:style {:display "flex" :flex-direction "flex-column"}}
  (when @loading
    {:disabled true {:style {:background-color "gray"}}})
  (when @demo
    {:class "demo-list"}))

This doesn't change the behavior of merge-props for existing code. (Unless people were relying on having (merge-props {}) throw an error?)

[Deraen]

Merged with some tuning (fixed :class collection handling) and I also added class-names utility function.