adding group option to specify multiple groups to run

adding a --group option to be able to specifiy multiple groups to run when -n option is used

````
parallel_tests test -n 5 --groups 3,4 --group-by filesize --type test
````
This because useful when building with travis which has 1.5 cores and recommends using 2 threads.
does changing this to `INT[, INT]` break the tests ?