Add Heroku support.

app.json

@@ -0,0 +1,16 @@
+{
+  "name": "NextAction",
+  "description": "Todoist API application to provide a auto-populated next action label",
+  "repository": "https://github.com/nikdoof/NextAction",
+  "keywords": ["python"],
+  "env": {
+    "TODOIST_API_KEY": {
+      "description": "Your Todoist API Key",
+      "required": true
+    },
+    "TODOIST_NEXT_ACTION_LABEL": {
+      "description": "The Todoist label to use for next actions.",
+      "value": "next_action",
+      "required": false
+    }
+}