Add soft signature failure support

Add a new trigger-signature-soft-failures option to allow soft signature
failures in Or rules.

Fixes #234

docs/Hook-Definition.md

@@ -22,6 +22,7 @@ Hooks are defined as objects in the JSON or YAML hooks configuration file. Pleas
 * `pass-file-to-command` - specifies a list of entries that will be serialized as a file. Incoming [data](Referencing-Request-Values.md) will be serialized in a request-temporary-file (otherwise parallel calls of the hook would lead to concurrent overwritings of the file). The filename to be addressed within the subsequent script is provided via an environment variable. Use `envname` to specify the name of the environment variable. If `envname` is not provided `HOOK_` and the name used to reference the request value are used. Defining `command-working-directory` will store the file relative to this location, if not provided, the systems temporary file directory will be used.  If `base64decode` is true, the incoming binary data will be base 64 decoded prior to storing it into the file. By default the corresponding file will be removed after the webhook exited.
  * `trigger-rule` - specifies the rule that will be evaluated in order to determine should the hook be triggered. Check [Hook rules page](Hook-Rules.md) to see the list of valid rules and their usage
  * `trigger-rule-mismatch-http-response-code` - specifies the HTTP status code to be returned when the trigger rule is not satisfied
+ * `trigger-signature-soft-failures` - allow signature validation failures within Or rules; by default, signature failures are treated as errors.
 
 ## Examples
 Check out [Hook examples page](Hook-Examples.md) for more complex examples of hooks.