delegate write :background option to Image.BackgroundColor

[hlindset]

Quick follow-up to #207, delegating the :background option of Image.write/3 to Image.BackgroundColor

[kipcole9]

Thanks @hlindset for this an all your PRs, really appreciated. You may have noticed I've been working on getting the library into shape towards a 1.0 release with lots of bug fixes, documentation updates, test coverage - that work is still ongoing but getting closer to completion. But it does mean CI isn't always green - although I think it will be stable again in the next 24 hours.

[hlindset]

Thanks @hlindset for this an all your PRs, really appreciated. You may have noticed I've been working on getting the library into shape towards a 1.0 release with lots of bug fixes, documentation updates, test coverage - that work is still ongoing but getting closer to completion. But it does mean CI isn't always green - although I think it will be stable again in the next 24 hours.

Happy to contribute!