fio2gnuplot: Deleting temporary files by default
fio2gnuplot: Managing temporary files in a better way

If gnuplot success at rendering, let's delete all temporary files.
If gnuplot fails at rendering, let's keep all temporary files.
If user don't want to run gnuplot, let's keep temporary files.
If user ask to keep the temporary files (option -k), let's always keep
them.