Support merged cells

[zackteo]

https://poi.apache.org/apidocs/dev/org/apache/poi/xssf/usermodel/XSSFSheet.html#addMergedRegion-org.apache.poi.ss.util.CellRangeAddress-

[zackteo]

Will currently take put the value of the merged-cell as the value of top most cell. (Other cell values are hidden but will appear when unmerged