Add buckets

[auduny]

It would be cool to be able to store numeric values into buckets.

In this case from a varnish log:

760 RxHeader     c X-VG-Client-RTT: 3793
 5146 RxHeader     c X-VG-Client-RTT: 320
 3064 TxHeader     b X-VG-Client-RTT: 2222
  772 RxHeader     c X-VG-Client-RTT: 703
 2388 RxHeader     c X-VG-Client-RTT: 2222
 4296 RxHeader     c X-VG-Client-RTT: 1457
 5464 RxHeader     c X-VG-Client-RTT: 2172

statpipe --buckets 1,2,3,5000