better identify :format segment of barista routes

barista nested resources will produce routes with :nestedmodel_format segments and thus do not pass the `val.path === '.:format'` condition. Using a regexp to match any :[a-z]+_format will allow nested resources to be used.
correct nested resource comment

Not documented, but Barista will nest resources with the syntax shown in the commit.
include snake-case multi-word in :format regex

ProductCategory would turn into :product_category_format and thus needs this modified regex.
Merge pull request #584 from jbielick/master

Enabling nested resources.