register figwheelApp to expo root if present and configured

[jgeraerts]

I've spend some time using figwheel-main inside expo. With some initial success I couldn't get around the fact that the figwheelApp() should be registered with expo itself as it does some extra initialization.

As such I've modified the bridge to take a configuration option and register it with expo if requested.

My main.js now looks like:

  Expo.KeepAwake.activate();
  //let ip = '10.32.128.182';
  let ip = '172.16.1.133';
  console.log("Firing up figwheel...");
  var figBridge = require("react-native-figwheel-bridge");
  figBridge.shimRequire(cljsExports);
  figBridge.start({appName:   "main",
                   expo:      true,
                   optionsUrl: `http://${ip}:9500/cljs-out/mobiledev/cljsc_opts.json`,
                   "closure-defines": {
                     "figwheel.repl.connect_url": `ws://${ip}:9500/figwheel-connect?fwbuild=mobiledev`
  },});