Add ability to markup validator description

Hi!
I'm using `config.markup = Apipie::Markup::Markdown.new` and I have my custom validator:

```ruby
class Apipie::Validator::StringQueryValidator < Apipie::Validator::BaseValidator
  def initialize(param_description, argument, options)
    super(param_description)
  end

  def validate(value)
    !!(value =~ /^[^&*\/\^]+$/)
  end

  def self.build(param_description, argument, options, block)
    if options[:validator] == :query
      self.new(param_description, argument, options)
    end
  end

  def description
    'Must have any characters except: `/` `&` `^` `*`'
  end
end
```

I'd like to make this description highlighted :smiley_cat: