Add support for top-level JSON array in payload

Detect if leading character in JSON payload is an array bracket.  If
found, decode payload into an interface{} and then save the results into
payload["root"].  References to payload values would need to reference
the leading, "virtual" root node (i.e. "root.0.name").

Fixes #215

test/hooks.json.tmpl

@@ -168,6 +168,22 @@
       ],
     }
   },
+  {
+    "id": "sendgrid",
+    "execute-command": "{{ .Hookecho }}",
+    "command-working-directory": "/",
+    "response-message": "success",
+    "trigger-rule": {
+      "match": {
+        "type": "value",
+        "parameter": {
+          "source": "payload",
+          "name": "root.0.event"
+        },
+        "value": "processed"
+      }
+    }
+  },
   {
     "id": "plex",
     "execute-command": "{{ .Hookecho }}",