Allow adding SVG in place of logo image (#976)

Usage =>
Params.label.iconSVG: "<your SVG as a string>"

layouts/partials/header.html

@@ -66,6 +66,8 @@
                 <img src="{{- site.Params.label.icon | absURL -}}" alt="" aria-label="logo"
                     height="{{- site.Params.label.iconHeight | default "30" -}}">
                 {{- end -}}
+                {{- else if hasPrefix site.Params.label.iconSVG "<svg" }}
+                    {{ site.Params.label.iconSVG | safeHTML }}
                 {{- end -}}
                 {{- $label_text -}}
             </a>