added an option to print temporal delimiters on subprosess print

[ressiwage]

So the idea is to plot vertical delimiters when a benchmarking program does print("cmdbench point") and then flushes stdout. I think that would be useful to check monotony of a program.
Imagine, for example, that you are decoding frames and you need to check that they are decoding in approximately same time. Then after each decoded frame you are doing the following:

printf("cmdbench point\n");
fflush(stdout);

and each delimiter will be plotted at printf time, see pictures below for example. Each vertical line is captured 'cmdbench point'.
also there is support for several iterations.

Please review and let me know what do you think.
P.S: I don't really know what to do with alpha, maybe you can figure out

[ressiwage]

@manzik