* Add ProxyRequests directive
This allows or prevents Apache httpd from functioning as a forward proxy server.
[See](https://httpd.apache.org/docs/current/mod/mod_proxy.html#proxyrequests).
The default action is off so I've set this a the default for this formula so it does not change the default behaviour.
* Add example pillar usage
This is done by split the interface pillar value instead of using a proper
list so it behaves exactly the same as before for simple 1 interface cases
(no need to refactor one's pillar files).
The resulting is something like:
Listen 1.2.3.4:80
Listen [2001🔤def💯:3]:80
<VirtualHost 1.2.3.4:80 [2001🔤def💯:3]:80>
...
