Use page_name instead of category/feed when formatting feed URL. Fixes #10
Use *.slug instead of page_name as latter includes relative URL. Fixes #10

The page_name variable includes relative path to the actual page_name
so it won't necessary match the feed file name if they are not in
the same path.

The category.slug and tag.slug variables instead are just a
shorthand for slugified category/tag name, which is what we need.