code: function Engine(opts) {
    opts = opts || {};
    this.ext = opts.ext || 'html';
    mkdirp = opts.mkdirp || mkdirp;
    this.version = require('./../package.json').version;
  }
docs: class Engine
  
  @param {String} opts: optional
  - ext: template file extension, files without this extension will be ignored.
  - mkdirp: temporary used for testing only, need to figure out how to mock required function using Sinon