Data visualizations from Clojure using Vega(-Lite) in the browser.

Waqi workflow is designed for the Clojurist who interacts with their
data by iteratively visualizing (parts of) it, with code and
resulting Vega-Embed simultaneously in view.

Users evaluate Vega/Vega-Lite specs (as Clojure maps) with `plot!`
to see their data visualized in the browser. If Waqi has not started
a server and established a WebSocket connection to a browser, it
calls `start-server!` (with or without a port argument) to do so. If
`plot!` cannot send a WebSocket message it will throw an error.
  
Architecture is as follows:

  [bare-bones HTML page on localhost]
            ^
            |
    (Vega(-Lite) specs,
         as JSON,
     sent via websocket)
            |
            v                  (Vega(-Lite) specs,
    [http-kit webserver] <---    as Clojure maps,     ---- [client]
                              sent via function call)