Add support for slashes in hook IDs

When matching variables in routes, gorilla/mux uses a default pattern of
"[^/]+", thereby prohibiting slashes in variable matching.  Override the
default pattern to remove this restriction.

See https://github.com/gorilla/mux/blob/v1.8.0/regexp.go#L50

Fixes #421

test/hooks.json.tmpl

@@ -184,6 +184,22 @@
       }
     }
   },
+  {
+    "id": "sendgrid/dir",
+    "execute-command": "{{ .Hookecho }}",
+    "command-working-directory": "/",
+    "response-message": "success",
+    "trigger-rule": {
+      "match": {
+        "type": "value",
+        "parameter": {
+          "source": "payload",
+          "name": "root.0.event"
+        },
+        "value": "it worked!"
+      }
+    }
+  },
   {
     "id": "plex",
     "execute-command": "{{ .Hookecho }}",