Support the 'percentThreshold' option

Added support for the statsd 'percentThreshold' option.

From https://github.com/etsy/statsd/blob/master/exampleConfig.js :

>percentThreshold: for time information, calculate the Nth percentile(s)
>                    (can be a single value or list of floating-point values)
>                    negative values mean to use "top" Nth percentile(s) values
>                    [%, default: 90]