Add snippet for new ActiveSupport::Concern module files

This pull request extends projectile-rails-expand-corresponding-snippet by adding a snippet for new files in app/*/concerns/ with a basic ActiveSupport::Concern template.

See http://api.rubyonrails.org/classes/ActiveSupport/Concern.html and https://signalvnoise.com/posts/3372-put-chubby-models-on-a-diet-with-concerns for details about concerns in Rails 4.
Maybe it would be useful to add 2 spaces before `extend ActiveSupport::Concern` to indent it properly?