fio2gnuplot: Option to select gnuplot's output dir

fio2gnuplot: Option to select gnuplot's output dir

This patch add the '-d' option to select where gnuplot shall render its
files.

When plotting big series of files, it's pretty anoying having all the
files in the current directory as there is many many of them.

So let's use this option to allow users specifying where all the
graphics shall be rendered