Support vips functions that operate on multiple images

[ryanlath]

Adds initial support for vips_FUNCTIONS that operate on multi files, like: vips_arrayjoin (this is the one I needed), vips_mosaic, vips_composite, vips_composite2.

// See tests for complete example
buffer, err := NewImageFrom(images, outputOptions).ArrayJoin(opts)

I didn't spend too much time on it, since I don't know if this is something the authors are open too, or whether this project is still active...

[klebekj]

Hi, is there any chance that this PR will be merged soon? Thanks.