Allow custom percentiles to report to Graphite

Graphite reporter now exports fixed percentiles only. This PR introduces the `GraphiteConfig.Percentiles` field to allow developers to set custom percentiles to export.

Keys formatting are backward compatible:
```
0.5 -> 50-percentile
0.75 -> 75-percentile
0.999 -> 999-percentile
0.01 -> 01-percentile
```