Issue #158: Use section title (with sectnums)

[eiswind]

I found that in helpers.rb the section_title was already defined with sectnum support.
I'm not sure if it is as simple as that, but I changed the section template to use this helper.

[ggrossetie]

Seems to be working as expected but you will need to update the tests:

• test/doctest/outline.html
• test/doctest/section.html

See: https://travis-ci.org/asciidoctor/asciidoctor-reveal.js/jobs/640352964

[eiswind]

Thanks for the hint :) There are tests...

[obilodeau]

Looking good! Awesome that helpers.rb already provided that.

Can you create a simple AsciiDoc file in examples/ that demonstrates the feature in a minimal way and with accompanying doctest.

I don't think README documentation is necessary since this is an upstream documented feature of the language that we will now support.

[eiswind]

just added a simple example ..

[obilodeau]

Thank you for taking the time to do a good PR!