Allow assertion of available options on a given select element

[deleugpn]

Closes #65.

This PR will allow assertions as follows:

$this->browse(function (Browser $browser) {
    $browser->visit(new HomePage)
        ->assertOptionAvailable('laravel-dusk', '1')
        ->assertOptionsAvailable('laravel-dusk', ['1', ''])
        ->assertOptionNotAvailable('laravel-dusk', '10')
        ->assertOptionsNotAvailable('laravel-dusk', ['10', 'unavailable']);
    });

On a select like this:

<select name="laravel-dusk">
    <option value="1">First</option>
    <option value="">Empty</option>
</select>

[lasselehtinen]

Awesome, thanks! 👍