Separate header and footer HTML from Content

I saw this done with Starkers and have used it ever since, and always
make this change when starting with JointsWP.

I separate the header.php file into its the vital HTML parts that don’t
change and the content markup parts like the opening DIVs and the logo
and menu.

This way I can easily make custom headers or footers for different
pages (and call them using get_template_part) without having to repeat
the HTML parts, which I can change from the single header-html.php file.