pt-query-digest --output json with Query_time histogram

Add Query_time histogram to pt-query-digest JSON output, not the actual chart but the values necessary to render the chart later, so the values for each bucket. The buckets are an array corresponding to these ranges:

(1us 10us 100us 1ms 10ms 100ms 1s 10s+)

So if the array has values (0, 10, 5, 1, 0, 0, 0, 0) then the query had 10 execution times in the 10us range, 5 in the 100us range, and 1 in the 1ms range.