Add a way to manage attributes set during run.

[AaronLasseigne]

There needs to be a way to cast incoming attributes to specific types. There should also be a way to set a default and do minor adjustments (like strip on String).

Reference: https://github.com/cypriss/mutations/wiki/Filtering-Input

[AaronLasseigne]

Ideas for syntax:

attrs do
  string  :name,
    allow_nil: true
  string  :email
  boolean :subscribed,
    default: true
end

or

string :name,
  allow_nil: true
string :email
boolean :subscribed,
  default: true