moved comments pending moderation

moved comments pending moderation

* comments: Comments pending moderation are now stored in the srcdir
  alongside accepted comments, but with a `._comment_pending` extension.
* This allows easier byhand moderation, as the "_pending" need
  only be stripped off and the comment be committed to version control.
* The `comment_pending()` pagespec can be used to match such unmoderated
  comments, which makes it easy to add a feed of them, or a counter
  indicating how many there are.
* Belatedly added a `comment()` pagespec.