Remove extra `/` from the links from `feed.xml`

In the `feed.xml` file, the links to the archive posts contain one extra
slash: `<link>http://wdrl.info//archive/[number]</link>`.

While this may not seem like an issue, in some contexts the extra slash
can be problematic (e.g.: when adding the link from Feedly to Pocket,
Pocket will just add `http://archive/[number]` instead of the correct
link).

This commit also adds the missing `grunt-contrib-copy` module.