Feature Request: allow filters on toc tag

[whicks1]

{% toc %} currently only renders the TOC based off of the {{ content }} var. I've run into scenarios where I needed to template in additional headings and metadata-generated content before or after the content as boilerplate HTML, that would still be nice to have in a table of contents. I'm supposing it might look like:

{%- capture key_concepts_heading -%}<h2>Key Concepts</h2>{% endcapture -%}
{%- capture license_heading -%}<h2>License</h2>{% endcapture -%}

{% toc | prepend: key_concepts_heading | append: license_heading %}

...
{{ key_concepts_heading }}
{{ page.key_concepts }}
{{ content }}
{{ license_heading }}
{{ page.license }}