Output per tag

Basically, I wanted to do here is to put arbitrary key_prefix for output json with `output_key_prefix` option like

    output_key_prefix response_time

so that fluentd outputs like

    {"response_time_max":xxx,"response_time_min":xxxx,....}

This enables me to use fluent-plugin-growthforecast in the same say with fluent-plugin-datacounter as I wrote on my blog http://blog.livedoor.jp/sonots/archives/25189820.html. 

To put the `output_key_prefix` option, I've noticed that I need to prepare `output_per_tag` option, and the `output_per_tag` requires `tag_prefix` option. So I also added these options in the same specification with `fluent-plugin-datacounter`.