Allow catalogue service filtering

[nvx]

The Consul catalogue service API endpoint allows filtering as per https://developer.hashicorp.com/consul/api-docs/catalog#list-services

It would be super handy if the services directive allowed passing through a filter. The other option would be to use Consul prepared queries, but it doesn't look like consul-template is able to query those either (#1137)

Implementing this would also likely allow closing of #33 - I note it mentions that you can't query by tag, but I'm guessing that is out of date as the API docs linked in the same comment makes it look like you can.