extract metric source and name from raw statsd metric name with functions in config

**motivation**

* say we have a metric `project_name.source.metric_name`
* `sourceRegex` assumes that we want to omit `project_name` from metric name when pushed to librato
* this is bad since librato has a single global namespace for all metrics

**solution**

introduce `measureName` and `measureSource` function that can be put in config

usage:
```
 librato: {
    email:  "me@example.com",
    token:  "bla bla",
    measureName: function (name) {
      return name.replace(/^([^\.]+)\.([^\.]+)\./, '$1.');
    },
    measureSource: function (name) {
      return name.match(/^[^\.]+\.([^\.]+)\./)[1];
    }
  }

```