Add anchor points to post list page

* post-year now has anchor point, you can click on it or input url with hash tag to jump to a specific year.
* See #39

layouts/_default/list.html

@@ -12,7 +12,7 @@
 		{{- end }}
 		{{- range .Data.Pages.GroupByDate "2006" }}
 		<div class="posts-group">
-			<div class="post-year">{{ .Key }}</div>
+			<div class="post-year" id="{{ .Key }}">{{ .Key }}</div>
 			<ul class="posts-list">
 				{{- range .Pages }}
 				<li class="post-item">