Add a "linkpost" format

Hey @mdo, I love your Poole setup but I was missing the ability to add link posts.
So here is a set of changes and additions I am proposing:

Addition to _config.yml:
  Add the ability to set a link post symbol like "→".

New file: _layouts/linkpost.html
  Takes care of linking the title to the link provided in the meta data and
  inserts the link post symbol.

Addition to index.html:
  Sets the correct link in the title for link posts and sets the post url to
  the post date in the homepage list of posts.

Addition to pubic/css/poole.css:
  Reset the look for the post date for when it is linking to the post page for
  link posts.

New example link post: _posts/2013-12-30-linkpost.md
  An example linkpost.