Add Strings::getStringBetween(...)

Usage example:

```php
$haystack = 'http://example.com/blog/some/page?id=1';
$domain = Strings::getStringBetween($haystack, 'http://', '/blog/'); // return 'example.com'
$args = Strings::getStringBetween($haystack, 'page?', null); // return 'id=1'
```