Add an option to toggle between shared strings and embedded strings

[afalkenhahn]

OpenXLSX already has code in XLCellValue.cpp that sets strings as embedded strings instead of using shared strings. Currently, this code is disabled, though. I think it would be nice if apps could specify whether strings should be added as shared strings or as embedded strings so an option to toggle between shared and embedded strings when setting cell values would be nice to have.

[aral-matrix]

putting this on the to-do list as of now.