Limit file size of reports

[rhcarvalho]

Reports can get quite large for long runs because they include data about every request, and in fact we could be plotting latency with significantly fewer and a limited amount of points using the downsampler built into Vegeta's plotting capabilities.

Something like this:

diff --git a/report.go b/report.go
index 96d9ea5..c6bbd10 100644
--- a/report.go
+++ b/report.go
@@ -92,7 +92,7 @@ func report(results []*RunResult) {
                }
        }
 
-       p := plot.New()
+       p := plot.New(plot.Downsample(1600))
        for i, res := range results {
                folderPath := res.Path
                name := res.Name