Add site logo option

exampleSite/config.toml

 
 [params]
   favicon = ""
+  site_logo = ""
   description = "The last theme you'll ever need. Maybe."
   facebook = ""
   twitter = "https://twitter.com/GoHugoIO"

layouts/partials/site-navigation.html

 <nav class="pv3 ph3 ph4-ns" role="navigation">
   <div class="flex-l justify-between items-center center">
     <a href="{{ .Site.Home.RelPermalink }}" class="f3 fw2 hover-white no-underline white-90 dib">
-      {{ .Site.Title }}
+      {{ with .Site.Params.site_logo }}
+        <img src="{{ . }}" class="w100 mw5-ns" alt="{{ $.Site.Title }}" />
+      {{ else }}
+        {{ .Site.Title }}
+      {{ end }}
     </a>
     <div class="flex-l items-center">
       {{ partial "i18nlist.html" . }}