New feature: set cell font style when saving with `cell.bold = true`, `cell.italic = true`, `cell.fontName = 'x'` or `cell.fontSize = x`
New feature: merge cells horizontally and vertically when saving with `cell.rowSpan = x` and  `cell.colSpan = x`
New feature: cells vertical alignement when saving with `cell.vAlign = 'x'`