code: function writeFile (file, data, options) {
      // return new Promise that will be resolved once data is written
      return new Promise((resolve, reject) => {
          // create write stream
          this.createWriteStream(file, options).then(stream => {
              // write data to stream and end
              stream.end(data)
              // resolve on finish
              stream.on('finish', resolve)
              // reject on error
              stream.on('error', reject)
          })
          // pass rejection
          .catch(reject)
      })
  }
docs: @function writeFile
  
  @param {string} file
  @param {Buffer|string} data
  @param {object} options
  
  @returns {Promise}