squid.conf and example.com
In certain circumstances the recipe uses example.com as a default hostname. I suggest not supplying such defaults. If the user doesn't give a value, don't supply anything; if a value is required, then fail if it isn't given.
The problem is that by supplying bogus defaults you could end up with very subtle errors. In particular, HTTP 1.0 clients would fail while HTTP 1.1 clients would work. While it's true that no browser would make a 1.0 request I believe it's equally true that many command-line tools still do.
Also, be aware that the recipe somewhat mimics Squid 2.7 defaults, but backwards. The recipe defaults visible_hostname from default_hostname, while Squid 2.7 defaults defaultsite from visible_hostname. (I don't know what 2.6 does.) It may not be worthwhile changing the recipe's behaviour, but it might be worth documenting the difference on the recipe's doc page.