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/main/folder-watcher.js

@@ -0,0 +1,54 @@
+const chokidar = require('chokidar')
+const log = require('./log')
+
+class FolderWatcher {
+  constructor ({window, state}) {
+    this.window = window
+    this.state = state
+    this.torrentsFolderPath
+  }
+
+  init () {
+    const torrentsFolderPath = this.state.saved.prefs.torrentsFolderPath
+    this.torrentsFolderPath = torrentsFolderPath
+    if (!torrentsFolderPath) return
+
+    const glob = `${torrentsFolderPath}/**/*.torrent`
+    log('Folder Watcher: watching: ', glob)
+
+    const options = {ignoreInitial: true}
+    this.watcher = chokidar.watch(glob, options)
+    this.watcher
+      .on('add', (path) => {
+        log('-- torrent added: ', path)
+        this.window.dispatch('addTorrent', path)
+      })
+  }
+
+  start (torrentsFolderPath) {
+    // Stop watching previous folder before
+    // start watching a new one.
+    if (this.torrentsFolderPath) {
+      this.stop()
+    }
+
+    const glob = `${torrentsFolderPath}/**/*.torrent`
+    log('Folder Watcher: watching: ', glob)
+
+    const options = {ignoreInitial: true}
+    this.watcher = chokidar.watch(glob, options)
+    this.watcher
+      .on('add', (path) => {
+        log('-- torrent added: ', path)
+        this.window.dispatch('addTorrent', path)
+      })
+  }
+
+  stop () {
+    if (!this.watcher) return
+    this.watcher.close()
+  }
+}
+
+
+module.exports = FolderWatcher