Add a feed to the blog

[dustym]

This change adds an Atom feed to the blog. The feed itself and the config are roughly based off the templates in the middleman 3.x tree

• add builder to the Gemfile
• add the feed.xml.builder template to blog/
• instruct middleman to render feed without a layout in config.rb
• add an author helper to config.rb. Defaults to “Staff” if no author is specified.
• add feed autodiscovery to blog templates
• add yield content block to the _head template (note the preceding whitespace here, it triggers a rebuild of some of the other pages)
• add the autodiscovery link to the contents-blog partial