Add support for popup windows

[biinari]

Add ability to use a popup window opened by the page.

Example Usage

On a page with an interaction that opens a window or tab:

<button id="open-popup" onclick="window.open('/popup', 'target', 'width=600,height=400')">Open a window</button>

or

<a id="open-popup" href="/popup" target="_blank">Open a tab</a>

Usage:

$page = visit('/');

$popup = $page->pendingPopup();
$page->click('#open-popup');

$popup->assertSee('Text in popup window');
// $popup interactions and assertions will wait for popup to open