Use supplied header ID for TOC rendering.

- Fixes #112 so that `#header {#header-id}` renders the TOC with
  `#header-id` instead of `#toc_1`.
I was going to comment that this method doesn't need to be exported, since it's not a part of implementing the `blackfriday.Renderer` interface, but then I learned that many other such methods that are already there are also exported. Perhaps another cleanup PR can unexport them (to make the blackfriday API slightly better).