Add option to modify command to be executed

The default command is "gnuplot -persist". On SmartOS, gnuplot
does not output to dumb terminal with -persist option.
cf. https://github.com/basho/basho_bench/issues/82
In such case, adding the option "-E gnuplot" will help.
Cosmetics: untabify and reorder
Merge pull request #106 from basho/ss-exec-command-option

Add option to specify exec command