customizable column width;
adding options to modify default font name and font size;
what if options is null?
add options.defaultCellBorders, defaultHAlign, defaultVAlign
customizable page margins, size, orientation;
add forceString;
temporary rowSpan and colSpan support;
update readme file;
add custom font color and row height and column width support;
fix vertical align is bottom problem;
added textRotation;
added page scale and sheet view option;
Merge pull request #3 from caiguanhao/master

feat(builder): Add options to customize the cells and the page.