kodo: add option to save benchmark plots

To be able to save/distribute the plots generated by
benchmarks/throughput/plot_throughput.py, a command-line argument is
added to specify the filename for the plots file.

To save the plots, call the script like this:
python2 plot_throughput.py --csv_file path/to/out.csv \
    --out_file /path/to/plots.pdf

When calling the script with the out_file argument, the plots are not
shown with the GUI.

Signed-of-to: Steinwurf ApS <info@steinwurf.com>
Signed-of-by: Martin Hundebøll <martin@hundeboll.net>
Only call pdf.savefig(transparent=True) if args.outfile defined