Update services func to support filtering by health check.

Allows one to query services by including an optional health status. See #65.

Currently only `"passing"` and `"any"` are supported but it should be easy to expand the available health check status options in the future once a consensus has been reached about what's required and how the filter syntax might look.
The general format is to start errors in this project with lowercase. I know it's not a valid sentence, but I would like to be consistent.

Also, you can use %q and avoid the escaping of quotes:

```go
fmt.Errorf("expected either %q or %q as health status", HealthyAny, HealthyPassing)
```