Names instead of IDs

I give names to all my docker containers, and in the fluentd data streams I would like to see those names rather than the meaningless IDs (for humans), especially since I then send the data to librato.

This is the first time I touch ruby, I don't know the first thing about the language, but with the help of StackOverflow I think I was able to make a meaningful improvement to your plugin. If you don't agree, no hard feelings :-)