Add className "has-background-*-color" to AB Container.

[shawndones]

I noticed that when adding a background color to AB container, the default WP behavior of adding the className "has-background-*-color" does not occur. It would be nice to see this implemented.

[jonschr]

+1 on this one. The use case here is that it would be nice to set default colors for text within blocks with a particular background color on both backend and frontend, something like this:

https://gist.github.com/jonschr/90e7703006a83966ffd26cd37695c727

This makes it faster and simpler to add content to sites.