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:  
I wonder if we shouldn't have the `Apipie.markup_to_html(params[:validator]).html_safe` here. The reasons:

1. through the JSON api that apipie provides, I would expect the non-html version
2. when adding new validator, one would always need to not forget to wrap that into the `Apipie.markup_to_html('...')`