Matcher

### Motivation

Avoid RSpec duplication when testing presence of ```PostcodeAnywhere::EmailValidation::Validator``` in ```ActiveModel``` objects

### Usage

When validating ```:email``` attribute:
```ruby
  it { should validate_email_with_postcode_anywhere }
```

With custom attribute.
```ruby
  it { should validate_email_with_postcode_anywhere.on_attribute(:email) }
```