Automatically create id for sub toc item

[samnung]

Add API for creating sub items for TOC + public API for creating ID from TOC title.

Bookspec for example:

Epuber::Book.new do |book|
  book.toc do
    toc.file 'text/intro', 'Intro' do
      toc.item 'Section'
    end
  end
end

It will create following items in toc:

<ol>
  <li><a href="text/intro.xhtml">Intro</a>
    <ol>
        <li><a href="text/intro.xhtml#section">Section</a>
    </ol>
  </li>
</ol>

Hash section will be generated from title. Example in Bade:

mixin section(title)
  h2.section(id: __helpers.id_for_title(title))= title