Add /via links for attribution

You can add a via: param to the front matter of any content type but I've only included it in the archtectypes for the types that are likely to need it. Close #18

archetypes/images.md

@@ -6,5 +6,7 @@ type: image
 # image is relative to /static/images/
 image: {{ .TranslationBaseName }}.jpg
 show_title: false
+# via: add a URL for attribution if it's not your image
+via: 
 tags:
 ---

archetypes/quotes.md

@@ -6,5 +6,7 @@ author:
 work:
 year:
 type: quote
+# via: add a URL for attribution
+via: 
 tags:
 ---

archetypes/videos.md

@@ -4,5 +4,7 @@ date: {{ .Date }}
 draft: false
 embed_url: 
 type: video
+# via: add a URL for attribution
+via: 
 tags:
 ---

layouts/partials/tags.html

@@ -2,5 +2,8 @@
   {{ range .Params.tags }}
   <li><a href="{{ relURL "/tags/" }}{{ . | urlize }}/">{{ . }}</a></li>
   {{ end }}
+  {{- with .Params.via -}}
+  <li><a href="{{ . }}">/via</a></li>
+  {{- end -}}
   <li><a href="{{ .Permalink }}" title="Permalink">#</a></li>
 </ul>