Few changes that allows the poi builder to write data into existing Workbooks or sheets starting at an arbitary position

This is useful if you want to merge your data into an Excel template. An example of this:

write to existing sheets this way and overwrite existing data there:

```java
SheetContext sheetCtx =
WorkbookContextFactory.useWorkbook(workbook).useSheet(sheet.getSheetName());

// move the cursor where the original cell is
sheetCtx.skipRows(sourceCell.getRowIndex()).setDefaultRowIndent(sourceCell.getColumnIndex());
// this will overwrite the original cell:
sheetCtx.text("overwrite!");
```