Set default service as github when service type empty (#40)

Set default service as github

Reviewed-on: https://gitea.com/gitea/changelog/pulls/40
Reviewed-by: Andrew Thornton <art27@cantab.net>
Reviewed-by: 6543 <6543@noreply.gitea.io>

config/config.go

@@ -54,5 +54,9 @@ func New(configPath string) (*Config, error) {
 		}
 	}
 
+	if cfg.Service == "" {
+		cfg.Service = "github"
+	}
+
 	return cfg, nil
 }