I have a use case where I would like to add custom content before the `</head>` tag. This change implements that. Users can add locally (either in their overriding theme or in the root directory) add /layouts/partials/overrides/header.html and the content is printed at that location.
I see multiple uses for this, after `<body>`, before `</body>`... The path in templates.Exists must be run through printf for some reason.
