index based on ${tag}

[cburling61]

I would like to see if it can be added to allow for the index to be set via using either the ${tag} from FluentD or some other variable.

In my case, I have logs coming in through FluentD that have a tag in them for the type of log they are. Example: network_log, http_log, path_log, etc. From my testing, the index placeholder in the configuration will not accept variable but must be hard coded if used. This enhancement would make it easier and less work than creating separate outputs for each one.