Add ReadDir function to list local files.

Includes documentation.

docs/layouts/shortcodes/fileindex.html

@@ -0,0 +1,16 @@
+<table style="width=100%">
+<th>Size in bytes</th>
+<th>Name</th>
+{{$dir := .Get "dir"}}
+{{ $url := .Get "baseurl" }}
+
+{{ $files := ReadDir $dir }}
+    {{ range $files }}
+			<tr>
+                <td>{{.Size}}</td>
+                <td>
+                    <a href="{{$url}}{{.Name | urlize }}"> {{.Name}}</a>
+                    </td>
+            </tr> 
+	 {{ end }}
+</table>