Use relative URL for favicon (#251)

This fixes sites which have the baseURL in a subdirectory.

Co-authored-by: Nick White <git@njw.name>

layouts/partials/site-favicon.html

 {{ if .Site.Params.favicon }}
-<link rel="shortcut icon" href="/{{ .Site.Params.favicon }}" type="image/x-icon" />
-{{ end }}
+<link rel="shortcut icon" href="{{ relURL ($.Site.Params.favicon) }}" type="image/x-icon" />
+{{ end }}