Use a custom parser for commands

The removes the dependency for `nom` as commands are now parsed with
its own tiny parser.

src/webhooks.rs

@@ -14,8 +14,6 @@ pub enum WebhookeyError {
     Unauthorized(IpAddr),
     #[error("Unmatched hook from `{0}`")]
     UnmatchedHook(IpAddr),
-    #[error("Could not find field refered to in parameter `{0}`")]
-    InvalidParameterPointer(String),
     #[error("Could not evaluate filter request")]
     InvalidFilter,
     #[error("IO Error")]