Implement secret functionality

In order to validate requests a field called `secret` has to be sent
containing a secret key which validates the request. A hook will be
executed only if the secret sent with the request matches the hook's
secret.

config.yml

@@ -1,17 +1,22 @@
 ---
 hooks:
   hook1:
-    action: "echo hookaction1"
+    action: /usr/bin/local/script_xy.sh
+    secrets:
+      - secret_key_01
+      - secret_key_02
     filters:
       match_ref:
-        pointer: "/ref"
-        regex: "refs/heads/master"
+        pointer: /ref
+        regex: refs/heads/master
   hook2:
-    action: "echo hookaction2"
+    action: /usr/bin/local/script_xyz.sh
+    secrets:
+      - secret_key03
     filters:
       match_ref:
-        pointer: "/ref"
-        regex: "refs/heads/master"
+        pointer: /ref
+        regex: refs/heads/master
       match_after:
-        pointer: "/after"
-        regex: "f6e5fe4fe37df76629112d55cc210718b6a55e7e"
+        pointer: /after
+        regex: f6e5fe4fe37df76629112d55cc210718b6a55e7e