cleanup
proper argument handling

 - for `-h`, print usage on stdout
 - abort if command is not given
exit with failed status if any rubies failed
abort iteration on SIGINT
improve colorization and header in verbose mode

Now there is no color if output is not to a tty.

Previous header was:

    [1.9.3-p194]: <command>
    *********************************************

It took two lines and displayed the command unnecessarily for each Ruby.
Now the header is:

    ===[1.9.3-p194]==============================