Add an option to use inline strings instead of shared strings.

Using inline strings reduces the amount of memory needed by Excel to process a file and therefore decreases the risk of OutOfMemory errors. Those errors are happening when the size of sharedStrings.xml becomes too big (around 1GB on a computer with 8GB of RAM).

Since the duplicated strings stored in sharedStrings.xml are not removed, this option just extends the current behavior.