Add support for author avatar and bio for multiple authors

[benjamincassidy]

Add support for multiple authors with author avatar image and author bio and make this available to selmer data for author.html templates.

Proposed config.edn:

{
  :authors [{:name "author name" :bio "author bio" :avatar "author.png"}]
}

[holyjak]

You can add whatever you want to the config and perhaps you can use :extend-params-fn - see http://cryogenweb.org/docs/customizing-cryogen.html - to pre-process it into a form suitable to be used in your selmer templates?