support "unknown" endRow and endColumn variables

[perNyfelt]

From https://github.com/AntoineKMS (extracted from issue #1 )
I am not sure If I should post it here, but I have another very small issue with this library. It's about the endRow, endColumn variables.
Would it be possible to have a default value that would just take as many rows and columns as there are in the sheet ?

It is very good to be able to tweak those parameters for when we only want certains parts of the sheet, but I think it would be good to have an option/ a default parameter to just take the whole sheet.

Thank you.

[perNyfelt]

I'll look at it. I think this is reasonably well supported in Apache POI for Excel but there is nothing like this for SODS (Libre/Open Office) so I'll have to do some manual probing for that which might be slow. Maybe a magical constant e.g. setting the endRow and endColumn to -1 or similar would be a decent way to support this.