Using different source images for different sizes

[sarahmonster]

Thank you for the excellent plugin!

I'd love to be able to provide a few different favicon sources for different sizes—what works at 16x16 isn't appropriate for 148x148, and vice-versa.

Is this possible to do with this plugin? It looks as though gatsby-plugin-manifest has this functionality, but I'd prefer to continue using gatsby-plugin-favicon if at all possible. I'd be happy to help by submitting a PR and/or writing some documentation. 😺