code: function getCacheKey()
  {
    // use the one from the context
    var directories = this.directories || this.defaults.directories;
  
    return resolveDir(directories)
      + ':'
      + getFiles.call(this, process.env).join(',')
      + ':'
      + resolveExts.call(this).join(',')
      ;
  }
docs: Generates cache key from the search directories
  and provided list of parsers
  
  @returns {string} - cache key