Allow square-bracket notation after anchor selector

[netbrothers-tr]

Currently the XPathExpressionDiscoverer allows selectors ending with /a only. This means being more specific by using the square-bracket notation is not supported. However, this would make the spider so much more powerful and we wouldn't even have to change a lot.

An example of the square-bracket notation could be the following.

//a[starts-with(@href, '/') or starts-with(@href, '$url')]

To allow this, spider could either be less strict about the selector argument (maybe replacing endsWith with a regular expression) or move the validation of the selector argument away from the constructor (to a protected function maybe), such that when extending the XPathExpressionDiscoverer you could override such validation method and have your own selector validation.