👋 Hey thanks for the great gem.

I'd like to propose supporting a conditional API similar to ActiveRecord's Callbacks.

Today, a conditional counter cache is implemented as follows:

class Product < ActiveRecord::Base
  belongs_to :category
  counter_culture :category, column_name: proc {|model| model.special? ? 'special_count' : nil }
end

class Category < ActiveRecord::Base
  has_many :products
end

I would like to propose the following shorthand:

class Product < ActiveRecord::Base
  belongs_to :category
  counter_culture :category, if: :special?
end

class Category < ActiveRecord::Base
  has_many :products
end

If you're amenable to that, I'd be happy to look into implementing the if/unless functionality (proposal: implementing the same API/semantics as ActiveRecord Callbacks), relevant specs, and interplay with the existing column_name. If not, no worries.