Add optional ip_filter to hook config

In order to allow or deny sources of requests the possibility to
configure a list of allowed or denied IP addresses was added as
described by the readme.

Closes #3

Cargo.toml

@@ -24,3 +24,4 @@ nom = "6"
 hmac = "0.10"
 sha2 = "0.9"
 hex = "0.4"
+ipnet = { version = "2.3", features = ["serde"] }