code: function rebuildBook(done) {
    // pause watcher while building book progress
    if (watcher) {
      watcher.end()
      watcher = undefined
    }
  
    // ensure output path is not exists otherwise book won't generated
    return del(path.join(outputPath)).then(paths => {
      buildBook().then(function() {
        // start watcher after built
        startWatch()
          .on('ready', function() {
            done()
          })
          .on('error', function(e) {
            console.error('ERROR: watch error... $s', e)
          })
      })
    })
  }
docs: kill watcher + remove/clean output dir + build book