Fix links with #comments fragment by adding id to post page

The hyperlinks to a permalink URL with a "comments" fragment don't
work: there is no "comments" fragment on the post page to target.

Add a "comments" id attribute to the list of comments, which renders
even when the list is empty.
Merge pull request #71 from mcary/fix-comments-anchor

Fix links with #comments fragment by adding id to post page