Add GobrakeSeverity option.

@dvdplm please review.
```
func InitGobrake() {
	glog.Gobrake = gobrake.NewNotifier(Cfg.GobrakeCfg.ProjectID, Cfg.GobrakeCfg.APIKey)
	glog.Gobrake.SetContext("environment", Cfg.EnvName)
        glog.GobrakeSeverity = glog.GobrakeWarningSeverity // Like this?
}
```