mosaicing arbitary bounding boxes

[spolloni]

Salut Vincent!

thanks for the wonderful suite of tools you're building here. I am looking for utils or code snippets in either rio-tiler-mosaic or rio-tiler that would help build an equivalent of the mosaic_tiler function, but accepting an arbitrary bbox rather than a (x,y,z) tuple. Any pointers?

Also, would such a function be a meaningful contribution to this repo? would be happy to take a stab at a PR if that interests you.