Expand unique validations with scoped and case-insensitive

[caspiano]

Allow uniqueness validations scoped by other fields on the model.
Additionally, allow case-insensitive uniqueness validations.

Usage:

class Post < Granite::Base
  connection mysql

  column id : Int64, primary: true
  column title : String
  column year : Int32

  validate_uniqueness(
     :title,
     scope: :year,           # Scope can also accept an array of fields
     case_insensitive: false # Default to strict case sensitivity
  )
end

[Blacksmoke16]

Related #358.

[crimson-knight]

I'm going to close this since it's tightly related to the RFC issue already cited. I think the recommendation makes sense and will naturally come about during the validation improvement process.