Allow symbol as values for `tokenize` of `LengthValidator`

This commit allows to specify a method name for `tokenize` of `LengthValidator`.

```
class Article
  include ActiveModel::Model

  validates_length_of :content,
    :minimum => 10,
    :message => "must be at least 10 words",
    :tokenizer => :tokenize_by_words

  def tokenize_by_words(text)
    text.scan(/\w+/)
  end
end
```

This makes the code easy to read, and is useful when the same tokenizer is used across the models.

Although it could be provided as a custom validator,  I would love to have this function in core ActiveModel. Please let me know what you think.