Working on watch-folder

Added chokidar to watch for folder changes; added folder-watcher;
passing state to delayedInit on main;  added default values for new
preferences; added “Auto add torrents” preference with its checkbox and
path selector; TODO: start/stop watching on preference change, start
watching on init, add dialog when trying to enable preference without a
torrents folder.

src/renderer/lib/state.js

@@ -121,7 +121,9 @@ function setupStateSaved (cb) {
       isFileHandler: false,
       openExternalPlayer: false,
       externalPlayerPath: null,
-      startup: false
+      startup: false,
+      autoAddTorrents: false,
+      torrentsFolderPath: ''
     },
     torrents: config.DEFAULT_TORRENTS.map(createTorrentObject),
     torrentsToResume: [],