Allow waitFor* actions when rendering.

[mjpieters]

When rendering a page, you sometimes need to wait for AJAX requests to complete. While using a sleep time can help, that method will easily fail if the network is slow or congested. Either expose the Pyppeteer page object or allow the caller to specify some waitFor* configuration.

For example, waitForNavigation() lets you specify a specific event to wait for, like networkidle0, which usually indicates all network-dependent JS code is done.

[kennethreitz]

similarly https://github.com/kennethreitz/requests-html/issues/99

[kennethreitz]

in master, you can now access the r.html.page attribute after running r.html.render().