Add a post's ID to the extracted metadata

I'm sure this will be useful for more than what I'm using it, but I need this in the metadata so I can reconstruct the Disqus identifiers correctly.

In my case, the identifiers look like `168 http://www.opensourcery.co.za/?p=168`, but the permalinks that are exported look like `/2009/03/20/pluck-out-an-old-revision-of-a-file-with-git-show/`, so I had no other way of even inferring the id.