Expand values exposed to the template

Exposed data structure passed to the template is richer so it now allows the following values to be used in the template:

```
{{.ID}}
{{.Service}}
{{.Tags}}
{{.Port}}
{{.IP}}
{{.Node}}
{{.}}
```

The default `{{.}}` value remains the same so this change will be backwards compatible. One could also simulate the default using the below values in the template string `server {{.Node}}_{{.ID}} {{.IP}}:{{.Port}}`.

This should allow this project to be used for nginx templates and also allow haproxy values like cookie name be set if needed.