feat: allow specifying favicon via configuration

We assume PNG MIME type, because, from my understanding, PNG is the main format
for favicons nowadays (even if they have an "ico" extension). Further
configuration options are needed to allow for specifying different MIME types.

layouts/partials/header.html

@@ -9,6 +9,10 @@
 	<title>{{ $title }} - {{ $siteTitle }}</title>
 	{{- end -}}
 
+	{{- if isset .Site.Params "favicon" -}}
+	<link rel="icon" type="image/png" href={{ .Site.Params.favicon }} />
+	{{- end -}}
+
 	<meta name="viewport" content="width=device-width, initial-scale=1">
 	{{ with .OutputFormats.Get "rss" -}}
 	{{ printf `<link rel="%s" type="%s" href="%s" title="%s" />` .Rel .MediaType.Type .Permalink $.Site.Title | safeHTML }}