* Adds `allow_url`, `block_url`, and `block_unknown_url` methods.
* Both allow_url and block_url accept wildcards.
* Any requests to blocked URLs will be blocked immediately.
* Blocked URLs take precedence over allowed URLs.
* By default, any requests to localhost and 127.0.0.1 are allowed.
* By default, any requests to unknown hosts will print a warning.
* `block_unknown_urls` causes unknown hosts to be silently blocked.
* `blacklisted_urls=` is deprecated in favor of `block_url`.
NetworkAccessManager was getting pretty complex, and adding something
that modifies or intercepts requests is becoming a common change.
This change introduces a chain of handlers which can modify a request or
return a response of their own.
To start with, this extracts the concerns of fixing missing content
types, setting custom headers, and intercepting blacklisted requests.
Joe Ferris