Option to choose \n or \n\n with new lines?

[hzlzh]

Same here #55

All the CSS properties build with new empty line like this screenshot, It will break gulp-sourcemaps format and fail.

https://github.com/reworkcss/css/blob/master/lib/stringify/identity.js#L44

Compiler.prototype.stylesheet = function(node){
  return this.mapVisit(node.stylesheet.rules, '\n\n');
};

Need option to choose \n or \n\n