Contribution of plugin logstash-filter-flatten_json

This filter plugin will flatten json data into strings, i.e.
```json
{
"key_1": "value_1",
"key_2": 2,
"key_3": {
"nested_key_1": "nested_value_1",
"nested_key_2": [1, 2, "a"]
}
}
```
the filter will enrich 'event' with following key-value pairs:
```ascii
key_1: "value_1"
key_2: 2
key_3.nested_key_1: "nested_value_1"
key_3.nested_key_2.0: 1
key_3.nested_key_2.1: 2
key_3.nested_key_2.2: "a"
```

This is very convenient if you have stats data from mongodb or docker-stats and you want to further process it or send to graphite server.

sources: https://github.com/mavlyutov/logstash-filter-flatten_json
tests: https://travis-ci.org/mavlyutov/logstash-filter-flatten_json