code: function getFiles (location, filterFn) {
    return new Promise((resolve, reject) => {
      const files = []
      klaw(location)
      .on('data', (item) => {
        if (!item.stats.isDirectory() && filterFn(item)) {
          files.push(item.path)
        }
      })
      .on('end', () => resolve(files))
      .on('error', reject)
    })
  }
docs: Walks over a location and reads all .js files
  
  @method getFiles
  
  @param  {String} location
  @param  {Function} filterFn
  
  @return {Array}