Worksheet Writer does not allow setting correct dimension ref value

[sarmstrongLN]

Hello,

Issue
Currently org.dhatim.fastexcel.Worksheet.flush() hardcodes the value of the dimension ref tag to "A1".
It means we cannot customise the value of this tag to correctly reflect the worksheet dimensions.

Suggestion
Add string property that can default to existing hardcoded "A1" value, but allow user to override this if required.