fields :title and :description to allow lambda

[jetienne]

Feature

When defining a :has_many field, it would be great to allow lambdas for :name and :description attribute. We could then display dynamic content like:

field :sent_invoices, 
         as: :has_many, 
         scope: lambda { query.sent }, 
         name: 'Sent Invoices', 
         description: lambda { "Total of #{query.sent.sum(:amount)}" }