add parameter 'output_style' for tagged output

The default output format of fluent-plugin-flowcounter looks like this:

    {"test_count":300, "test_bytes":3660, "test_count_rate":5, "test_bytes_rate":61, "service1_count":180, "service1_bytes":7260, "service1_count_rate":3, "service1_bytes_rate":121}

With this patch and configuration "output_style tagged", you'll get the following outputs:

    {"tag": "test", "count":300, "bytes":3660, "count_rate":5, "bytes_rate":61}
    {"tag": "service1", "count":180, "bytes":7260, "count_rate":3, "bytes_rate":121}

I'd like to have this output format for easier post processing.  Can you merge this change?

Thanks,