Abort requests to external resources or given domains

- [x] add ``--no-externals`` option to [disallow requests to 3rd party resources](http://ariya.ofilabs.com/2013/06/web-page-without-external-resources.html)
- [x] add ``--allow-domain`` parameter to whitelist given domain when in ``--no-externals`` mode (CDNs)
- [x] add ``--block-domain`` parameter to disallow requests to given domain