Support configuring custom keys via method invocation

[jorgemanrubia]

This adds support to generate keys via a method name passed as a symbol:

class Person
  kredis_list :names, key: :generate_key

  private
    def generate_key
      "some generated key"
    end
end

Currently, the only way to customize keys is via a lambda, lilke key: ->(p) { "person:#{p.id}:names_customized" }. The problem with that approach is that, when you want to invoke a method to determine the key, you need to make that method public. This is also a more concise way of covering a pretty common use case: using an instance method to determine keys.