feat: add background support on DIV and A elements.

[euskadi31]

[vvo]

Hi! Thanks for the contribution.

Once you are done (maybe already the case?), can you:

• provide some context (what are you trying to do? Is there already a linked issue? Maybe we can make this a plugin or a recipe, should this be in core?)
• document the API in the github issue (we will then see if it's a good fit, how do we make it to the readme)
• remove the built file from the commit (I will build it when releasing if necessary)

[euskadi31]

I added a example for background lazyload feature.

[vvo]

@euskadi31 Thats very nice, can you just update the test according to my comment?

[vvo]

For this PR, we still need:

• [ ] good documentation
• [ ] remove builds from commits

I can do that, no problem, if you want to do it that's good also. Thanks a lot for contribution