Supplementary view offsetting is ignored

[CallMeSH]

It is possible on iOS 13 to offset a supplementary view.
It took me a while to understand how they work and to obtain the result I wanted.

Unfortunately the library didn't support the offsets.
I have written the code needed to make my views work (top and bottom).
I also spent some time trying to modify the sample code of HeaderFooterViewController.swift to try several alternative configuration and I'm confident it's very close to what iOS 13 is doing in most situations.

PR is following this issue.

Thanks!