Experimental TOC rendering support

See #63

This implementation does NOT render TOC at a specific tag, but
just prepends the TOC portion in front of the content (but after
front-matter). The TOC syntax might be more difficult. Still
considering options.
Use [TOC] as TOC placeholder

Only bare [TOC] (case insensitive) inside standalone <p> tag is
supported now. Don't know what the world would want; let's open
things when they are requested.
Merge branch 'toc'

Fix #63