Feature: Support window.close()

Under #40 you added support for window.open(), but if a script invokes window.close(), for example in response to a click on a "Proceed" button, the window doesn't disappear.

https://github.com/ChromeDevTools/devtools-protocol/issues/146#issuecomment-460563159 might be useful.

Review issue #40 and its implementation of window.open() first, then follow the linked Chrome DevTools Protocol discussion for window.close(). The work is done when a script calling window.close(), such as one triggered by the Proceed button, causes the preview window to disappear.

Written by the indexing model from the issue text.