use optimized writer for writing out xlsx so as to support large files

[chfw]

optimized writer is a better fit for pyexcel-xlsx.

trigger: pyexcel-webwares/Flask-Excel#16
hint: http://stackoverflow.com/questions/21328884/openpyxl-writing-large-excel-files-with-python
doc: http://openpyxl.readthedocs.io/en/2.3.3/optimized.html

[chfw]

@rotten, to evaluate before its release:

pip install https://github.com/pyexcel/pyexcel-io/archive/master.zip
pip install https://github.com/pyexcel/pyexcel-xlsx/archive/master.zip

[rotten]

Thanks, I'll give it a try tomorrow.

[rotten]

It seems to work. I'm not sure it is faster than the xlsxw solution, but it seems to be fairly stable and so far I haven't been able to crash it with large data sets. :-) Thanks.

[chfw]

So it seems the following promise of "write-only" from openpyxl is valid.

It is able to export unlimited amount of data (even more than Excel can handle actually), while keeping memory usage under 10Mb.