support nested name

Support nested name in order to process nested data like:
```
{"dstat": {"memory_usage": {"used": 12434, "free": 325425}}}
```

Configuration is below:
```
<match metrics>
  type growthforecast
  gfapi_url http://growthforecast.local/api/
  service   service1
  section   metrics1
  name_keys dstat.memory_usage.used,dstat.memory_usage.free
</match>
```