Allow RegEx constraints for new parameters

[splashsky]

The next big priority item is to accept a RegEx constraint for router parameters, via a method that can be chained onto a route method. This would look something like this:

// Single parameter
Router::get('/hello/{name}', function ($n) {
    return "Hello, $n!";
})->where('name', '[A-Za-z0-9]+');

// Multi parameter
Router::get('/hello/{name}/age/{num}', function ($n, $a) {
    return "Hello, $a yo named $n!";
})->where(['name' => '[A-Za-z0-9]+', 'num' => '[0-9]+']);

This ensures the user can have the level of control they want, while keeping it concise and clean.