Add proxyrequests directive (#144)

* 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

apache/vhosts/proxy.tmpl

     'LogFormat': site.get('LogFormat', '"%h %l %u %t \\\"%r\\\" %>s %O"'),
     'CustomLog': site.get('CustomLog', '{0}/{1}-access.log'.format(map.logdir, sitename)),
     
+    'ProxyRequests': site.get('ProxyRequests', 'Off'),
     'ProxyPreserveHost': site.get('ProxyPreserveHost', 'On'),
     'ProxyRoute': site.get('ProxyRoute', {}),
 } %}
     SSLCertificateChainFile {{ site.SSLCertificateChainFile}}
     {%   endif %}
     {% endif %}
-
+    ProxyRequests {{ vals.ProxyRequests }}
     ProxyPreserveHost {{ vals.ProxyPreserveHost }}    
     {% for proxy, proxyargs in vals.ProxyRoute|dictsort|reverse %}
     {% set proxyvals = {

pillar.example

       # RedirectTarget: 'http://www.example.net'
 
       # if template is 'proxy.tmpl'
+      # ProxyRequests: 'On'
       # ProxyPreserveHost: 'On'
       # ProxyRoute:
       #   my sample route: