Support for break-after: avoid, break-before: avoid

[Stouffi]

After using the polyfill to achieve successfully a document preview (content splitted on A4 sheet), I was trying to perform page breaks to make better presentation like forcing page break after some element, etc.
However I was unable to avoid a break after a specified element like a header which I would not place at the very bottom of a sheet and then have the content following on the next sheet.
Using break-after: avoid or break-after: avoid-region did not help. So I guess that this is not supported yet.
Reading other issues and regarding the activity of this repo. It may be hard to get this to work in the future. But anyway I open this issue and we'll see what will happen.