localhost and localhost. are valid domains, but give ValidationFailure

```python
import validators
validators.domain('localhost') # -> ValidationFailure
validators.domain('localhost.') # -> ValidationFailure
```
refs
* https://stackoverflow.com/questions/2532053/validate-a-hostname-string

edit: or does 'domain' mean 'fully qualified domain name'? ([fqdn](https://github.com/ypcrts/fqdn/blob/develop/fqdn/__init__.py#L65))
then 'localhost' is invalid
