yagp: Move index.html into html.h

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
yagp: Move the thumbnail row/cell html to html.h

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
yagp: Make images per page and row runtime settable

You can now optionally pass -p and or -r options. -p sets the number
images to show per page and -r sets the number of images per row. images
per page must be wholly divisible by images per row.

The defaults if not set remain at 16 images per page with 4 images per
row.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>