Add format param to the mergeCells() API
Use the orignal cell format if no format is specified
Fix numFmt: don't assume that all custom numFmtId starts from 164
Fix the reading of style.xml

Save all the formats <xf>s in order even they are duplicated.
Add read support for indexedColors

Some old .xlsx files use indexed colors
Worksheet: Fix constructor initialization list order
Fix SharedString loading bug