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.yaml.tmpl

@@ -97,6 +97,18 @@
           name: "app.messages.message.#text"
         value: "Hello!!"
 
+- 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
   trigger-rule:
     match: