Add bookmarks support

[seanh]

These would be posts that appear in the posts list, but that link to something you've posted elsewhere (a GitHub Gist, Stack Overflow answer, etc). They'd have some sort of "external link" icon next to them in post listings. The external link icon and also the date could link to the permalink page for the bookmark post, which would also be used by jekyll-feed.

Bookmark posts could have a different category or type front matter variable, which would trigger different templates being used to render them.