Add sort and grouping functions for publish date and param of Page

`GroupBy` is modified to allow it to receive a method name argument for
example `Type` as its first argument. It is only allowed to call with
a method which takes no arguments and returns a result or a pair of
a result and an error.

The functions discussed at #443 are also added

- `ByPublishDate`: Order contents by `PublishDate` front matter variable
- `GroupByPublishDate(format, order)`: Group contents by `PublishDate`
  front matter variable formatted in string like `GroupByDate`
- `GroupByParam(key, order)`: Group contents by `Param` front matter
  variable specified by `key` argument
- `GroupByParamDate(key, format, order)`: Group contents by `Param`
  front matter variable specified by `key` argument and formatted in
  string like `GroupByDate`. It's effective against `time.Time` type
  front matter variable

docs/content/templates/list.md

@@ -178,6 +178,15 @@ your list templates:
     </li>
     {{ end }}
 
+### Order by PublishDate
+
+    {{ range .Data.Pages.ByPublishDate }}
+    <li>
+    <a href="{{ .Permalink }}">{{ .Title }}</a>
+    <div class="meta">{{ .PublishDate.Format "Mon, Jan 2, 2006" }}</div>
+    </li>
+    {{ end }}
+
 ### Order by Length
 
     {{ range .Data.Pages.ByLength }}
@@ -219,7 +228,7 @@ Can be applied to any of the above. Using Date for an example.
 ## Grouping Content
 
 Hugo provides some grouping functions for list pages. You can use them to
-group pages by Section, Date etc.
+group pages by Section, Type, Date etc.
 
 Here are a variety of different ways you can group the content items in
 your list templates:
@@ -252,6 +261,48 @@ your list templates:
     </ul>
     {{ end }}
 
+### Grouping by Page publish date
+
+    {{ range .Data.Pages.GroupByPublishDate "2006-01" }}
+    <h3>{{ .Key }}</h3>
+    <ul>
+        {{ range .Pages }}
+        <li>
+        <a href="{{ .Permalink }}">{{ .Title }}</a>
+        <div class="meta">{{ .PublishDate.Format "Mon, Jan 2, 2006" }}</div>
+        </li>
+        {{ end }}
+    </ul>
+    {{ end }}
+
+### Grouping by Page param
+
+    {{ range .Data.Pages.GroupByParam "param_key" }}
+    <h3>{{ .Key }}</h3>
+    <ul>
+        {{ range .Pages }}
+        <li>
+        <a href="{{ .Permalink }}">{{ .Title }}</a>
+        <div class="meta">{{ .Date.Format "Mon, Jan 2, 2006" }}</div>
+        </li>
+        {{ end }}
+    </ul>
+    {{ end }}
+
+### Grouping by Page param in date format
+
+    {{ range .Data.Pages.GroupByParamDate "param_key" "2006-01" }}
+    <h3>{{ .Key }}</h3>
+    <ul>
+        {{ range .Pages }}
+        <li>
+        <a href="{{ .Permalink }}">{{ .Title }}</a>
+        <div class="meta">{{ .Date.Format "Mon, Jan 2, 2006" }}</div>
+        </li>
+        {{ end }}
+    </ul>
+    {{ end }}
+
 ### Reversing Key Order
 
 The ordering of the groups is performed by keys in alpha-numeric order (A–Z,