Add option to make header content overlaps sidebar

[Andrey Selemenev]

Hi @arkon. Thank you for your great work on this component.
Can you add optional ability to make the header that overlaps sidebar, like in my screenshot:

Or if some workaround exists can you help with it?
For example here https://stackblitz.com/angular/mggrmqgvbqn?file=app%2Fsidenav-fixed-example.ts I can add top/bottom gap for sticky header/footer

[arkon]

That's out of scope of this package, but the source code is available in this repo for that demo, which just relies on basic styling.