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/models/selection.go

@@ -25,9 +25,9 @@ type ServerSelection struct {
 	// Cyberghost
 	Group string `json:"group"`
 
-	// Mullvad, PureVPN
-	Countries []string `json:"countries"`
-	Cities    []string `json:"cities"`
+	Countries []string `json:"countries"` // Mullvad, PureVPN
+	Cities    []string `json:"cities"`    // Mullvad, PureVPN, Windscribe
+	Hostnames []string `json:"hostnames"` // Windscribe
 
 	// Mullvad
 	ISPs  []string `json:"isps"`