Add TOC widget

[pintergreg]

In my setup, I wanted to have a table of content on the sidebar, so I made a widget based on the post_toc.html.
The widget uses the same title.

This might be useful for others.

Using in the example site:

sidebar: true
widgets:
  - toc

[Vimux]

That's cool, but I can't merge it. Our built-in widgets support caching. I don't plan to add non-cacheable widgets to the default set.