Possibility to add placeholder image url

[JuhQ]

Would be neat if the imageCommand could take in a passed prop to use a predefined image url, instead of the current https://example.com/your-image.png. This way we could render some pre-existing placeholder image, if for some reason the user does not provide their own.

Seems that the current implementation exists in https://github.com/andrerpena/react-mde/blob/master/src/commands/markdown-commands/imageCommand.tsx, I'm not familiar enough with the project to make such PR.