[Feature Request] Add support for custom favicons (browser tab icons) in the .gotty file

[CoconutMacaroon]

I'd like to be able to set the tab favicon to a custom .png/.ico in the .gotty config file. You can already set the tab title, so setting the tag icon/favicon seems reasonable too. I can set it manually by using a custom index.html, but I'd prefer to be able to do it directly from the .gotty config file. I'd like to be able to do something like this:

favicon = "/my_favicon.png"

Can this be implemented, or, alternatively, would anyone mind if I sent a PR? I'd be willing to add this myself.

[sorenisanerd]

Sure, a PR would be very welcome :)