feature: add svg traces

[kpfromer]

Description

• This adds the ?trace parameter for generating SVG image traces using the potrace library.
  • other options should work as well, like resize, width, and height (I have not tested the rest though). By default, svg trace will create a trace of a full-sized image, which is slow. I recommend resizing the image down using resize and width or height.
• Also optimizes the inline data string for SVGs using mini-svg-data-uri - gatsby-plugin-sharp does this as well. https://webpack.js.org/loaders/url-loader/#svg

This implementation is heavily based on the one used in gatsby-plugin-sharp (seen here)

Images

Example Image:

Example Img Component:

Trace:

Todo

• [ ] How should potrace options be passed? More require query arguments?
• [ ] General note: should images have quality/progressive options?

Thanks again, @cyrilwanner, for making this awesome image tool suite. It's much appreciated!