How to send data in the `custom` field?

First, thank you for this library!

Second, I have a bunch of contextual data I'd like to send in the report, but I don't see an any way to do this.  From https://rollbar.com/docs/api/items_post/ :
```
    // Optional: custom
    // Any arbitrary metadata you want to send. "custom" itself should be an object.
    "custom": {},
```

What would you recommend?

