Windscribe API and more servers filter options, fixes #197 (#282)

- Use Windscribe API to fetch servers information
- More data on servers about region, city and hostname
- Add optional server filters with `REGION`, `CITY` and `HOSTNAME` csv environment variables

internal/updater/cyberghost.go

@@ -101,6 +101,20 @@ func getCyberghostGroups() map[string]string {
 	}
 }
 
+func mergeCountryCodes(base, extend map[string]string) (merged map[string]string) {
+	merged = make(map[string]string, len(base))
+	for countryCode, region := range base {
+		merged[countryCode] = region
+	}
+	for countryCode := range base {
+		delete(extend, countryCode)
+	}
+	for countryCode, region := range extend {
+		merged[countryCode] = region
+	}
+	return merged
+}
+
 func getCyberghostSubdomainToRegion() map[string]string { //nolint:dupl
 	return map[string]string{
 		"af": "Afghanistan",