Update engines.js

allow underscore engine to receive options parameter(s) for _.template function.

an example:
  _.templateSettings = {
    evaluate    : /<%([\s\S]+?)%>/g,
    interpolate : /<%=([\s\S]+?)%>/g,
    escape      : /<%-([\s\S]+?)%>/g
  };
i.e use this.template('_myfile.xml','myfile',null,templateSettings)
to get default behavior of _.template. 

useful when dealing with extra template inside source. like maven vars