Add GitHub "edit" button (#157)

layouts/_default/single.html

@@ -15,7 +15,8 @@
   <article>
       {{if .Title}}<h1>{{ .Title }}</h1>{{end}}
       <p class="meta">
-          Last updated {{if ne .Date .Lastmod}}{{ .Lastmod.Format "January 2, 2006" }}{{else}}Unknown{{end}}
+          Last updated {{if ne .Date .Lastmod}}{{ .Lastmod.Format "January 2, 2006" }}{{else}}Unknown{{end}}. 
+          {{ partial "github.html" . }}
       </p>
       <ul class="tags">
           {{ range (.GetTerms "tags") }}

layouts/partials/github.html

@@ -0,0 +1,3 @@
+{{if $.Site.Data.config.enableGitHubEdit}
+<a href="{{$.Site.Data.config.GHlink}}/{{ .Path}}" rel="noopener">Edit Source</a>
+{{end}}