Add support for specifying a custom audit model via the audited options

[muhammad-usman-anwar]

This helps with achieving targeted behavior for each audited model. Plus can be used to split the audited data across.

Example Usage

To specify a different audit model for a specific audited model, create a new class that
inherits from Audited::Audit:

class CustomAudit < Audited::Audit
  def some_custom_behavior
    "Hiya!"
  end
end

Then provide the class name in options:

class User < ActiveRecord::Base
  audited class_name: "CustomAudit"
end