Add an option `in` for the numeric validation of the model attributes.

Hi,

I sent a message on the Core list without pertinent answers. Thus, this pull request will able to refactor this code:

```ruby
class Person
  validates_numericality_of :age, greater_than_or_equal_to: 1, less_than_or_equal_to: 100
end
```
in:
```ruby
class Person
  validates_numericality_of :age, in: 1..100
end
```

Although, we can use ```validates_inclusion_of```, It will be more consistent to define the same kind of options for one same attribute.

Thanks