Support URL option for Page.waitForNavigation

[imiric]

Currently the URL option for waitForNavigation is unused.

Previously it wasn't a major priority, but now that waiting for navigation within the same document works, we need a way to distinguish same document and new document navigations.

Suggested solution

Playwright handles this with the URL option, and it makes sense for us to do the same.

[imiric]

Ah, I suppose this is a duplicate of #107, just with a different API. We should ensure they both expose the same implementation.