feat: implement `vips_image_get_string` for `VipsImage`

[zanmato]

This can be used to fetch various string metadata such as the vips-loader, which in turn can be used to determine the image type.

See: https://www.libvips.org/API/current/libvips-header.html#vips-image-get-string

Fixes #43

[afajl]

Hi, whats the status on this? Can we help out to get this merged in any way?