option for country code validation in helper

validate that the proper country code is part of a number if the appropriate option is passed, so you can do:

```ruby
class AustralianHelpfulHome < ActiveRecord::Base
  attr_accessor :phone_number
  validates_plausible_phone :phone_number, :country_code => "61"
end
```

and added some missing specs to ensure that you can use multiple options on the validator like so

```ruby
class BigHelpfulHome < ActiveRecord::Base
  attr_accessor :phone_number
  validates_plausible_phone :phone_number, :presence => true, :with => /^\+\d+/, :country_code => "33"
end
```