Add support for parsing GDAL ghost area

[kylebarron]

https://gdal.org/en/stable/drivers/raster/cog.html#header-ghost-area

Ideally we'd have a way to inject support for this.

In #7 @geospatial-jeff warned that it may not be worth supporting, but I think it would be pretty easy to add support because it's generic binary parsing and doesn't rely on any of the existing tiff tag parsing.