Minify XML output

[mattparks]

I was testing xlnt output against the output from libxlsxwriter, when writing the same data the xlnt xlsx file was always larger. After doing some digging it appears that the internal xml files had whitespace enable (they are not minified).

It appears that I only had to change the follow line to minify the xml outputs:

xlnt/source/detail/serialization/xlsx_producer.cpp

Line 182 in f302601

current_part_serializer_.reset(new xml::serializer(current_part_stream_, part.string()));

This is the change I made on my fork: https://github.com/mattparks/xlnt/commit/6c32ca8745843aec77f58e7a904b8fca5a62fca5

[tfussell]

I didn't minify it yet as it makes debugging more difficult, but it makes sense to allow the developer to choose. I'll make this configurable.