Feature Request : Send Image Message

ability to send an message object with these format.

```
json
{
  "type": "image",
  "originalContentUrl": "https://example.com/original.jpg",
  "previewImageUrl": "https://example.com/preview.jpg"
}
```


base from Line documents.
[https://developers.line.biz/en/reference/messaging-api/#image-message]()