Add --launcher-msg option to pass to desktop app

Summary:
Allows the launcher to provide messages to the user. Currently, in the form
of the red notification bar at the bottom. This is just meant as a temporary measure
during the alpha to have some clearly noticeable way of getting the user's attention.

I consider removing this a blocker for the release as this mechanism is not well
suited for this in many ways.

The current use case for this is providing a warning if a cached version is used
instead of the requested one, e.g. "Could not fetch requested Flipper version 'deadbeef', using cached version instead."

Reviewed By: jknoxville, priteshrnandgaonkar

Differential Revision: D14073687

fbshipit-source-id: 85630347027063103315eeb1286731fe2478e261

static/setup.js

@@ -76,6 +76,7 @@ module.exports = function(argv) {
   config = {
     ...config,
     updaterEnabled: argv.updater,
+    launcherMsg: argv.launcherMsg,
   };
 
   return {config, configPath, flipperDir};