Use signature field for verification

Instead of looking for a "secret" field hmac is used. Therefore the
raw payload is hashed with all secrets consecutively in order to
validate its content. If the content is certified the established
behaviour is pursued..

README.md

@@ -55,11 +55,12 @@ 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: Optional 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 }}`
+- 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.
 - secrets: List of secrets.
 - filters: List of filters.
 
@@ -81,7 +82,6 @@ Whereas `<config_dir>` depends on the platform:
 - Windows: `{FOLDERID_RoamingAppData}`
 
 # TODOs
-## Use `lazy_static` or `once_cell` for compiled regexes
 ## Use `clap` to parse command line arguments
 ## Implement the functionality to reply to certain webhooks
 ## Configure rocket via config.yml