Add HashiCorp & Community download pages

## HashiCorp:

- [Envconsul](https://github.com/hashicorp/envconsul)
- [Consul Template](https://github.com/hashicorp/consul-template)
- [Consul Replicate](https://github.com/hashicorp/consul-replicate)
- [Consul Web UI](https://github.com/hashicorp/consul/tree/master/ui)

## Community

- [Confd](https://github.com/kelseyhightower/confd)
- [Consulate](https://github.com/gmr/consulate)
- [Crypt](http://xordataexchange.github.io/crypt/)
- [git2consul](https://github.com/ryanbreen/git2consul)
- [docker-consul](https://github.com/progrium/docker-consul)
- [fsconsul](https://github.com/ryanbreen/fsconsul)
- [registrator](https://github.com/progrium/registrator)

/cc @armon @mitchellh