Arbitrary header fields in commands

Adopt the parser to be able to parse header fields.

README.md

@@ -55,16 +55,26 @@ Configuration syntax is YAML and has to be done in following order:
 
 Right now there is only the configuration parameter for hooks, here
 each hook has to be configured, It contains following fields:
-- command: String for a command to be executed when all filters
-  match. Pointers ([RFC 6901](https://tools.ietf.org/html/rfc6901)) to
-  JSON fields may be used to be replaced with data from the JSON data
-  with `{{ /field/pointed/to }}`. Further `{{ event }}` and `{{
-  signature }}` are valid variables as they contain the values from
-  the regarding header fields of the http request.
+- command: A command to be executed if a filter matches
 - signature: Name of the HTTP header field containing the signature.
 - secrets: List of secrets.
 - filters: List of filters.
 
+### Command
+To pass data to a command following two different methods can be used.
+
+#### JSON Pointers
+Use JSON pointers ([RFC 6901](https://tools.ietf.org/html/rfc6901))
+point to values of a JSON field from the JSON data.
+
+Example: `{{ /field/pointed/to }}`.
+
+#### Header
+Use values from header fields sent with the HTTP request.
+
+Example: `{{ header X-Gitea-Event }}`.
+
+### Filter
 Each filter must have following fields:
 - pointer: pointer to the JSON field according to [RFC
   6901](https://tools.ietf.org/html/rfc6901)