[Feature Request] Add specifying image by name support to DropletCreateImage

[dm3ch]

It would allow addressing self-built images via a name instead of ID, which would make working with DO easier.

P.S. Slug is not available to self-built images, as far as I found in docs.

[gaurav123-4]

@dm3ch @DigitalOceanY Please assign me the issue so that i Can start working on it