Allow for custom date formats in articles

I am accustomed to dates in the mm/dd/yyyy format. This change allows for specifying `:date_format` in the rackup config when creating a server. So for example if my Rakefile is changed to include a date like `article = {'title' => title, 'date' => Time.now.strftime("%m/%d/%Y")}.to_yaml` (as in Dorothy), I can parse it correctly with specifying `set :date_format, "%m/%d/%Y"`.

Thanks for creating such a concise blog engine!