Add background mode back

[frederickfogerty]

Before you submit:

• Please search through the existing issues (both open AND closed) to see if your feature has already been discussed. Github issue search can be used for this: https://github.com/imgix/react-imgix/issues?utf8=%E2%9C%93&q=is%3Aissue
• Please take a moment to find out whether your idea fits with the scope and aims of the project

Is your feature request related to a problem? Please describe.
I would like to set images as the background image of components such as div so I can place other items within this element.

Describe the solution you'd like

<Imgix type="bg" src="..."><Heading>Blog Header</Heading></Imgix>

We could make this work by requiring width and height to be specified, as determining this ourselves is too error prone.

[Sandstedt]

This is essential for our project. So we need to use an old version of react-imgix currently. Could use object-fit in newer projects, but when we need to support older browsers, it's not possible.

[frederickfogerty]

Due to the community desire to have this reimplemented, this is happening! I'll post any updates here when I have them 👍

[frederickfogerty]

We're getting very close now! Track the progress over in #236.

[frederickfogerty]

🎉 Released in 8.5.0. Thanks to everyone for their support and @adamraider for contributing to the PR!