add a switch to turn off sidebar

Sometimes we don't need a sidebar for some pages, e.g. a biography page. Add a switch to control whether show the sidebar.

One can control whether show sidebar for a specific page or article by define a ``sidebar`` meta. Like

```
Title: Biography
Author: Awesome one
Slug: biography
Sidebar: off
```

Values can be ``on``, ``off``, ``true``, ``false``, case insensitive, internally they are converted to lower case ``on`` or ``off``.