Allow custom key prefix

Most of our graphite plugins support a configurable key prefix. A good example is logster (see example here: https://github.com/etsy/logster/blob/master/bin/logster#L73) I'd like to duplicate the same functionality in this plugin.

- Currently the prefix is hardcoded to `elasticsearch`. If the prefix configuration is not specified, I would like it to default to `elasticsearch`. No breaking behavior change here.
- Otherwise, swap in whatever the configured prefix name.

For a configuration key name, I was thinking `metrics.graphite.prefix`. Does that sound reasonable?

I'm going to hack up a commit for this feature since we need this. Is this something that could be merged into mainline?

Thanks!

Blake