Fix host validation and short status output
Except for GitLab.com all hosts must be listed in the configuration file. Previously all Git remotes were assumed to be GitLab servers.
This change adds host validation to the API client to prevent API calls being made to remotes that are not GitLab servers, like GitHub.com.
Furthermore, error output from host validation is suppressed in the short status output to for more convenient automation.
Closes #10 (closed)