Improve the "cjsTranslate" option

[rubennorte]

If I got it right, the current behavior of the option cjsTranslate is to convert all the js files found in the directory. In projects with lots of external dependencies this option makes the optimization of modules too slow.

I think it would be nice to extend this options with more possible values:

{
  cjsTranslate: true, // current behavior
  cjsTranslate: false, // current behavior
  cjsTranslate: 'all', // all modules, like "cjsTranslate: true"
  cjsTranslate: 'optimized' // only optimized modules and its dependencies
  cjsTranslate: ['main.js', 'cjsmodules'] // list of files or directories to be converted
}