Add yet another RabbitMQ Exporter

Remove leftover from old  behavior

Next release

Update docs for k8s pods sd

Add docs for ignoring and new group_* semantics.

re-order exporters list

So the differences with listed one (https://github.com/kbudde/rabbitmq_exporter) are these:
 - prometheus_rabbitmq_exporter is not standalone daemon but a plugin for RabbitMQ itself. 
 - it also exports Erlang VM stats.

Also I tried to stay compatible with rabbitmq_exporter as much as possible (probably added a couple of labels)

List unofficial Erlang client

Aerospike exporter

Adding BIG-IP exporter

Add unless to precedence table

Document holt_winters()

Fixes #363

Adding in link to Rancher exporter to docs

Update EC2 SD docs

FIxing docs to match available labels in EC2 sd.

[https://github.com/prometheus/prometheus/blame/master/retrieval/discovery/ec2.go](https://github.com/prometheus/prometheus/blame/master/retrieval/discovery/ec2.go)

CNCF/Prometheus announcement blog post.

Blog post about when to use the new varbit chunks

Delete duplicate word

Fix bullet list and other nits in ShowMax post.