Add announcement feature

If there's a message returned by the given remote URL, then it will
show up for the user.

Useful in situations where the auto-updater is not working, or if
there's a security issue.

main/announcement.js

@@ -0,0 +1,27 @@
+module.exports = {
+  init
+}
+
+var electron = require('electron')
+var get = require('simple-get')
+
+var config = require('../config')
+var log = require('./log')
+
+var ANNOUNCEMENT_URL = config.ANNOUNCEMENT_URL +
+  '?version=' + config.APP_VERSION +
+  '&platform=' + process.platform
+
+function init () {
+  get.concat(ANNOUNCEMENT_URL, function (err, res, data) {
+    if (err) return log('failed to retrieve remote message')
+    if (res.statusCode !== 200) return log('no remote message')
+
+    electron.dialog.showMessageBox({
+      type: 'info',
+      buttons: ['OK'],
+      title: 'WebTorrent Desktop Announcement',
+      message: data.toString()
+    })
+  })
+}

main/index.js

@@ -5,6 +5,7 @@ var electron = require('electron')
 var app = electron.app
 var ipcMain = electron.ipcMain
 
+var announcement = require('./announcement')
 var config = require('../config')
 var crashReporter = require('../crash-reporter')
 var handlers = require('./handlers')
@@ -91,6 +92,7 @@ function init () {
 }
 
 function delayedInit () {
+  announcement.init()
   tray.init()
   handlers.install()
   updater.init()