Include sticker info in !source

Currently, the `source` command works with message ID, content, attachments, and embeds, but not stickers (which are of course now released for custom uploading), so I think that should also be supported.

Example:
```json
{
  "id": "859858669486342176",
  "content": "",
  "attachments": [],
  "embeds": [],
  "stickers": {
    "id": "1234",
    "name": "pogcow"
  }
}
```