[fluentd] Add separate aggregator chart

The two use cases for _Fluentd_ are very different in implementation and as such rather than trying to shoehorn both into a single chart it would make sense to have a separate `fluentd-aggregator` chart. I already have a [fluentd-aggregator](https://github.com/stevehipwell/helm-charts/tree/master/charts/fluentd-aggregator) chart in my repo and would be happy to create a PR to add it to this repo and deprecate mine.

It would also be useful to have an aggregator _Docker_ image to handle the common aggregation endpoints; an aggregator isn't as concerned with size and if the plugins are layered on top of the base image it would keep the layers easily portable. I've created my own [image](https://github.com/stevehipwell/fluentd-aggregator) for my _Helm_ chart, although the chart will work fine with the base _Fluentd_ image (by default it outputs to null and has stdout debugging available), but an official version would be better.

I'd like to call out @MrD2 for his work on PR https://github.com/fluent/helm-charts/pull/55 for the _Fluentd_ collector chart, this would be intended to augment this not replace it.