add statsd exporter mode

[Nomon]

In some conditions it makes more sense for nomad to push changes via statsd protocol and have nomad-exporter use that data to update the prometheus metrics instead of calling out to API. Add statsd mode that listens to statsd protocol and rewrites those metrics to equivalent nomad-exporter names and labels that are then exposed via /metrics.

see:
https://github.com/prometheus/statsd_exporter