Add a favicon option to easily control link rel=icon

[darobin]

Specs are nicer when they are prettier, and a neat way to improve them is to throw in a favicon. See:

• https://globalprivacycontrol.github.io/gpc-spec/
• https://darobin.github.io/garuda/
• https://nytimes.github.io/std-cat/
• https://darobin.github.io/pup/

This adds a favicon core configuration that can take either a link to an image, or a Unicode character that gets inlined as an SVG icon.

[marcoscaceres]

I think if we go with a single letter, we might want to do something similar to Safari (which does this automatically):

I.e, center the letter and place it in box that assures high contrast?

[marcoscaceres]

@darobin, let us know what you would like to do with this one?

[marcoscaceres]

Closing due to inactivity. Feel free to rebase and send again, @darobin.