Add support for model validation

perhaps something like `is_(not_)valid_with`. To replace the following:

```ruby
specify_with('foo', 'bar', 'baz' )      { is_expected.to be_valid }
specify_with('foo', 'bar', :invalid)    { is_expected.not_to be_valid }
```

Replacement might look like:
```ruby
is_valid_with     'foo', 'bar', 'baz'
is_not_valid_with 'foo', 'bar', :invalid
```