Add flag to check if toc is empty

[njanakiev]

First of all thanks for the great project! I think an useful feature might be to have a flag to see if the table of contents is empty. Perhaps something in this direction:

{% if not toc.empty %}
<h2>Table of Contents</h2>
{% endif %}

This could be useful for short articles that do not require a table of contents.

[rathboma]

Yes! I definitely need something like this. I have an empty 'on this page' block otherwise.