Inject event's fields into data_points

It would be great to add a capability to get some fields matched a given pattern from event object and put them into data points. It is useful in the cases when your logs may contain some metrics, but their structure is dynamic/not known so it cannot be defined strictly inside the config of the logstash instance.
The pull request adds some "dynamicity" to output configuration.
This can be done by the new configuration options given in the next example:
```
output {

}
```