[Feature request]: Add Gatsby image compat with Imgix images

[madeleineostoja]

Borrowing an idea from gatsby-source-prismic, it would be great if we could add a compat layer between Prismic's Imgix processing and gatsby-image, so a) we don't have to download and transform images locally (huge drain on build time), and b) we can work with rather than against the default Imgix processing (see: #177).

The logic to do so is pretty straightforward, see how gatsby-source-prismic does it

[ohlr]

@seaneking your link is broken