Add an ignore flag for the ToC

[jfhbrook]

For example, on a blog with some extra pages, the extra pages (such as "archive", "about", etc.) will show up in the ToC.

Maybe the page.json can look like:

{
  "toc": {
    "index": false
  }
}

Commentary appreciated!

[optikfluffel]

+1 :)

[indexzero]

Not sure if this is resolved in blacksmith@1.0.0, but after a rewrite it's probably not relevant.