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.
Same thing here about the lowercase. I also think we can throw a more general error here like:

```go
"expected 1 or 2 arguments, got %d"
```