Gatsby: Support "traced placeholder" generation in gatsby-plugin-sharp

[KyleAMathews]

Ran across on Twitter this really cool demo of a generating outlines of images and using them as placeholders while loading the image:

https://twitter.com/mikaelainalem/status/918213244954861569

It'd be fun if we could build this into our image processing pipeline

Following this thread which suggested using a unix tool named potrace. There's also a little-used JS port of potrace.

If anyone wants to experiment with this, it'd be a really cool addition to Gatsby!

[benjaminhoffman]

I think this has been merged yeah? Can close issue?

[KyleAMathews]

Indeed!

[hackhat]

This should be added to gatsby-remark-images to be consistent, otherwise some images will be blurred, others will have svg.