Histogram reporter

This release adds the hist[buckets] reporter to the report command which computes and prints a text based latency histogram for the given buckets. Here's an example of using it.

$ cat results.bin | vegeta report -reporter='hist[0,2ms,4ms,6ms]'
Bucket         #     %       Histogram
[0,     2ms]   6007  32.65%  ########################
[2ms,   4ms]   5505  29.92%  ######################
[4ms,   6ms]   2117  11.51%  ########
[6ms,   +Inf]  4771  25.93%  ###################

Thanks to @bgentry for stirring up the discussion on this feature!