Allow injecting cookies into a webview

We requiere a way to set cokies for http-requests made from inside a web view. Since the `document.cookie` browser API can be used inside a web view, an `executeScript` or `eval` method on webview may suffice (See #667). However, since the page may send requests before the page is fully loaded, we would requiere some kind of mechanism to execute a script as early as possible. Therefore we may also want to consider an explicit support for this feature, like `webView.setCookie(k,v);`.