lib/Preview.js
function addFlag() {
  const filename = arguments[0];
  const postfix = [].slice.call(arguments, 1).join('.');
  return filename.replace('.html', '.' + postfix + '.html');
}
function addFlag() {
  const filename = arguments[0];
  const postfix = [].slice.call(arguments, 1).join('.');
  return filename.replace('.html', '.' + postfix + '.html');
}
function addFlag ( ) { const filename = arguments [ 0 ] ; const postfix = [ ] . slice . call ( arguments , 1 ) . join ( '.' ) ; return filename . replace ( '.html' , '.' + postfix + '.html' ) ; }
Appends a list of flags separated by a '.' to a filename.
Appends a list of flags separated by a . to a filename .