Special syntax to sum subfields

[tshemsedinov]

After implementing #19 it would be great to have something like in examples below but need more research about syntax, it may be different:

  const sheet = new Sheet();
  sheet.cells['item1.price'] = 100;
  sheet.cells['item2.price'] = 200;
  sheet.cells['item3.price'] = 300;

  // Instead of: sheet.cells['total'] = '=item1.price + item2.price + item3.price';
  sheet.cells['total'] = '=sum(item1, item2, item3).price';