now you can explicitly name your 100% percentile via config

**motivation:**

Let's say that I have named all my timers as `prefix.metric_name` and have `percentThreshold:[95,99]` set up in config.

This backend will name metrics in librato as `prefix.metric_name`, `prefix.metric_name.95` and `prefix.metric_name.99`. 

Now let's say that I want to create a composite mean metric in librato of all 100% percentile metrics that match `prefix.*` like so `mean(s("prefix.*", "*"))`. This will include all `prefix.*.9*` metrics.

----

Given this patch I'm able to explicitly name the 100% percentile metric. 

Example:

With `defaultPercentileName: '100'` and  `percentThreshold:[95,99]` in config.js I'll now have the following metrics available in librato: `prefix.metric_name.100`, `prefix.metric_name.95` and `prefix.metric_name.99`.