Http proxy support

[marksy-91]

Quick fix for node.js server using tabletop running behind a corporate proxy.
Providing the httpProxyUrl option will enable the node.js server to resolve the request through the proxy.

E.g.

Tabletop.init({
    key: "key",
    callback: function = (data, tabletop) => {console.log(data)},
    httpProxyUrl: "http://username:password@proxy:proxyport",
  });