New function for mutate filter

it merges two fields or arrays or hashes in the first one
Strings fields will be converted in array, so 
- array + string will work
- string + string will result in an 2 entry array in dest_field
- array and hash will not work

example 1: add a multiples tags as a csv
mutate { 
               # tags_field = "tag1,tag2,tag3"
                split => [ "tags_field", "/" ]          
                merge => ["@tags", "tags_field"]
            }

example 2: add a json field to @fields
json {
            # json_field = "{"data1":"Value1","data2":"Value2"}"
            json_field => json_data
}
mutate { 
                merge => ["@fields", "json_data"]
            }