Add optional ip_filter to hook config

In order to allow or deny sources of requests the possibility to
configure a list of allowed or denied IP addresses was added as
described by the readme.

Closes #3

config.yml.example

@@ -2,6 +2,10 @@
 hooks:
   hook1:
     command: /usr/bin/local/script_xy.sh {{ /field2/foo }} asdfasdf
+    signature: X-Gitea-Signature
+    ip_filter:
+      allow:
+        - 127.0.0.1/31
     secrets:
       - secret_key_01
       - secret_key_02
@@ -11,6 +15,10 @@ hooks:
         regex: refs/heads/master
   hook2:
     command: /usr/bin/local/script_xy.sh asdfasdf
+    signature: X-Gitea-Signature
+    ip_filter:
+      deny:
+        - 10.10.10.0/22
     secrets:
       - secret_key_01
       - secret_key_02
@@ -20,6 +28,7 @@ hooks:
         regex: refs/heads/master
   hook3:
     command: /usr/bin/local/script_xyz.sh
+    signature: X-Gitea-Signature
     secrets:
       - secret_key03
     filters: