Add multiple --prepare support

[iamsauravsharma]

Add support for multiple --prepare option. Corresponding --prepare option will be used for command for benchmarking according to position. If --prepare option is less than benchmark command then all other remaining benchmark command will use last prepare option ( This logic may not be best logic to implement out). If prepare option is more than command for benchmarking then extra --prepare option will not be used
Part of #216

[sharkdp]

Thank you for your contribution.

If --prepare option is less than benchmark command then all other remaining benchmark command will use last prepare option ( This logic may not be best logic to implement out). If prepare option is more than command for benchmarking then extra --prepare option will not be used

I would solve both problems by issuing an error message if --prepare has not been specified exactly once or exactly N times. This way, we don't have to make these decisions. Either, we use one preparation-command for all benchmark-commands, or we have a 1:1 mapping of preparation-commands to benchmark-commands.

[iamsauravsharma]

@sharkdp How would you like to raise error currently I am directly raising error by calling error function which is simple. And any suggestion over error message which was raised I think this is not best error message for this error

[sharkdp]

Thank you very much. I added some comments.

[sharkdp]

Thank you for the updates!