ui-shared: add support for header/footer options when embedded=1

When embedded=1, cgit used to ignore the header and footer options.
But honoring these options when embedded=1 makes it possible to "frame"
the html fragment generated by cgit with any kind of static content,
i.e. it should become easier to integrate cgit with site-specfic
layouts.

Original-patch-by: Mark Constable <markc@renta.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>