Add an option to enable Rouge's plaintext lexer

[ashmaroli]

Use case

~~~ruby
class Foo
  def bar
    puts 'Hello'
  end
end
~~~

~~~text
class Foo
  def bar
    puts 'Hello'
  end
end
~~~

~~~
class Foo
  def bar
    puts 'Hello'
  end
end
~~~

Actual Output

<div class="language-ruby highlighter-rouge">
  <div class="highlight">
    <pre class="highlight">
      <code>
        <span class="k">class</span> <span class="nc">Foo</span>
        <span class="k">def</span> <span class="nf">bar</span>
        <span class="nb">puts</span> <span class="s1">'Hello'</span>
        <span class="k">end</span>
        <span class="k">end</span>
      </code>
    </pre>
  </div>
</div>

<pre>
  <code class="language-text">
    class Foo
      def bar
        puts 'Hello'
      end
    end
  </code>
</pre>

<pre>
  <code>
    class Foo
      def bar
        puts 'Hello'
      end
    end
  </code>
</pre>

Expected Output

I expected all three code-blocks to have similar encapsulation similar to kramdown-1.17 so that the 3 code-blocks render with similar appearance at the front-end:

<div class="language-ruby highlighter-rouge">
  <div class="highlight">
    <pre class="highlight">
      <code>
        <span class="k">class</span> <span class="nc">Foo</span>
        <span class="k">def</span> <span class="nf">bar</span>
        <span class="nb">puts</span> <span class="s1">'Hello'</span>
        <span class="k">end</span>
        <span class="k">end</span>
      </code>
    </pre>
  </div>
</div>

<div class="language-text highlighter-rouge">
  <div class="highlight">
    <pre class="highlight">
      <code>
        class Foo
          def bar
            puts 'Hello'
          end
        end
      </code>
    </pre>
  </div>
</div>

<div class="highlighter-rouge">
  <div class="highlight">
    <pre class="highlight">
      <code>
        class Foo
          def bar
            puts 'Hello'
          end
        end
      </code>
    </pre>
  </div>
</div>

Summary

Since Rouge's plaintext lexer was voluntarily removed for kramdown-2.0, it would be helpful if there was an option to enable it according to a Document's use case.