feat(template): update Atom template

BREAKING CHANGE: Now you can use `link_blog` and `rss_excerpt` for your Atom feed:

- `link_blog`: Enable this settings to make your feed Daring Fireball-like link blog. This basically change your <link> value to external link you defined in your posts.
- `rss_excerpt`: Use post excerpt instead of full content for RSS feeds, just like WordPress RSS feed output option
feat(post): update posts for discontinued external links
Bump version to 24.0.8