Add options to specify header/footer margin

[yoursdearboy]

The PR adds options --header-margin and --footer-margin to specify top/bottom margin for header/footer (not to be confused with document top/bottom margin).
This is useful if you have a dynamic header and don't know its size to use --margin-top option. So omitting --margin-top and using --header-margin instead will get the job done. They can be used simultaneously.
Probably it solves this issue #2940.

I wonder whether it can be solved better than this way. Maybe options can be renamed to better reflect their meaning.

[assendk]

Any chance to this be resolved?
The custom/dynamic headers or footers heights are very important for the pdf generation. Especially if is possible have a different footer on the last page. I know that by design we don't know the height of the last page so it's impossible to align some content/footer at the bottom part of the last page, near the footer.
Probably instead of --header-margin is better to be named --header-height
Thank you

[ashkulz]

Why can't you use the spacing?

[assendk]

Sorry, its my fault. At the moment I'm unable to implement this layout and align the "content2" div to the bottom of the second page. Is it possible to be done with your amazing app?
https://i.imgur.com/Dj6x0nK.png