Session cookie key is now configurable

- Added [web] cookie.auth.key to configuration
- Inbucket generates a random key if none is configured
- Added [default] default.domain to be reference by SMTP and POP3
  configs
- Updated default/sample config files

config/config.go

@@ -39,6 +39,7 @@ type WebConfig struct {
 	TemplateCache bool
 	PublicDir     string
 	GreetingFile  string
+	CookieAuthKey string
 }
 
 // DataStoreConfig contains the mail store configuration
@@ -348,6 +349,15 @@ func parseWebConfig() error {
 	}
 	webConfig.GreetingFile = str
 
+	option = "cookie.auth.key"
+	if Config.HasOption(section, option) {
+		str, err = Config.String(section, option)
+		if err != nil {
+			return fmt.Errorf("Failed to parse [%v]%v: '%v'", section, option, err)
+		}
+		webConfig.CookieAuthKey = str
+	}
+
 	return nil
 }