Add option to disable generation of sourcemaps

[21brains-zh]

I added the option to disable sourcemaps.

Example bundle.config.js:

module.exports = {
  bundle: {
    main: {
      styles: ['./public/less/styles.less','./public/less/bootstrap.less','./public/less/font-awesome/font-awesome.less'],
      options: {
        transforms: {
          styles: less // pipe(s) of style transforms
        },
        maps: false
      }
    }
  }
};

[chmontgomery]

released in v2.18.0