Add support for header/footer image buffers

[mohd-akram]

I noticed there's no way to insert a header/footer image via a buffer. I'm thinking perhaps the lxw_header_footer_options struct can have three new fields, image_left_size, image_center_size and image_right_size. If one is non-zero, its corresponding image field is read as a buffer instead of a filename.

[jmcnamara]

I noticed there's no way to insert a header/footer image via a buffer.

I stopped short of adding that because I thought that I would wait until someone requested it. And also because the interface was going to be a bit complicated. I still don't have a strong idea about what it should look like. I'll need to think about it for a while, or at least get some time to think about it.