Support so called conjunction filters

This introduces thee so called conjunction filters and therefore
restructures the configuration file. The most obvious changes from an
users perspective are that the `filters` field was renamed to `filter`
and can, from now on, only support a single filter at first
level. Thats why now different filter types are implemented, please
consult the readme for further information on their usage.

To reflect the changes the readme file is updated as well as the
example config file contained in this repository.

This is related to #8

config.yml

@@ -9,8 +9,8 @@ hooks:
     secrets:
       - secret_key_01
       - secret_key_02
-    filters:
-      match_ref:
+    filter:
+      json:
         pointer: /ref
         regex: refs/heads/master
   hook2:
@@ -22,19 +22,24 @@ hooks:
     secrets:
       - secret_key_01
       - secret_key_02
-    filters:
-      match_ref:
-        pointer: /ref
-        regex: refs/heads/master
+    filter:
+      and:
+        - json:
+            pointer: /ref
+            regex: refs/heads/master
+        - json:
+            pointer: /after
+            regex: f6e5fe4fe37df76629112d55cc210718b6a55e7e
   hook3:
     command: /usr/bin/local/script_xyz.sh
     signature: X-Gitea-Signature
     secrets:
       - secret_key03
-    filters:
-      match_ref:
-        pointer: /ref
-        regex: refs/heads/master
-      match_after:
-        pointer: /after
-        regex: f6e5fe4fe37df76629112d55cc210718b6a55e7e
+    filter:
+      or:
+        - json:
+            pointer: /ref
+            regex: refs/heads/master
+        - json:
+            pointer: /after
+            regex: f6e5fe4fe37df76629112d55cc210718b6a55e7e