Swift String Extension: Can convert to url

canConvertToURL

```swift
extension String {

    /// - Returns a Bool to determine if a string can be converted to a valid url
    func canConvertToURL() -> Bool {
        if let url = URL(string: self) {
            return UIApplication.shared.canOpenURL(url)
        } else { return false }
    }
    
    /// - Returns a URL if the string can be converted to a valid url,
    /// and a `nil` if it failed
    func convertToURL() -> URL? {
        if let url = URL(string: self) {
            if UIApplication.shared.canOpenURL(url) { return url }
            else { return nil }
        } else { return nil }
    }
    
    var url: URL? {
        return convertToURL()
    }
    
}
```