Replace command parameters with values

To create a minimalistic parser, nom is used to identify and replace
parameters given in the command field.

For clarity the `action` field for hooks was renamed to `command`.

README.md

@@ -1,8 +1,8 @@
 # Webhookey
-Webhookey basically is a webserver listening for requests as for
-example sent as gitea's webhooks. Further, Webhookey allows you to
-specifiy rules which are matched against the data received to trigger
-certain actions.
+Webhookey is a webserver listening for requests as for example sent by
+gitea's webhooks. Further, Webhookey allows you to specifiy rules
+which are matched against the data received to trigger certain
+actions.
 
 ## Build
 
@@ -55,12 +55,15 @@ 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:
-- action: optional string for the action to be executed when all
-  filters match
-- secrets: list of secrets
-- filters: list of filters
+- 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 }}`
+- secrets: List of secrets.
+- filters: List of filters.
 
-Each filter has to have following fields:
+Each filter must have following fields:
 - pointer: pointer to the JSON field according to [RFC
   6901](https://tools.ietf.org/html/rfc6901)
 - regex: regular expression which has to match the field pointed to by
@@ -88,3 +91,4 @@ Whereas `<config_dir>` depends on the platform:
 ### Authentication features
 ### Secure cookies?
 ## Parameterize fields
+## Use proptest or quickcheck for tests of parsers