privoxy: add launchd plist

Closes Homebrew/homebrew#25672.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/privoxy.rb

@@ -25,4 +25,29 @@ class Privoxy < Formula
     system "make"
     system "make install"
   end
+
+  plist_options :manual => "privoxy #{HOMEBREW_PREFIX}/etc/privoxy/config"
+
+  def plist; <<-EOS.undent
+    <?xml version="1.0" encoding="UTF-8"?>
+    <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+    <plist version="1.0">
+    <dict>
+      <key>KeepAlive</key>
+      <true/>
+      <key>Label</key>
+      <string>#{plist_name}</string>
+      <key>WorkingDirectory</key>
+      <string>#{var}</string>
+      <key>ProgramArguments</key>
+      <array>
+        <string>#{sbin}/privoxy</string>
+        <string>#{etc}/privoxy/config</string>
+      </array>
+      <key>RunAtLoad</key>
+      <true/>
+    </dict>
+    </plist>
+    EOS
+  end
 end