Refactor slim/html5 templates using Helpers

I’ve refactored Slim templates for HTML5 backend in order to separate most of logic into helpers, make templates more clear, simpler and “DRY”.

This PR includes seven previous (not yet merged) PRs and also some future changes/fixes. I’ll rebase it later to make it more clear, so consider this PR just as a preview for review. The most important commit is [this](https://github.com/jirutka/asciidoctor-backends/commit/d7dce1b40b7e9479ab84253113194f2619780eb9).

I depends on changes in the upcoming Asciidoctor 1.5.2.
Well, renaming attribute and specifically `id` is not the best example; [this one](https://github.com/asciidoctor/asciidoctor/blob/master/lib/asciidoctor/abstract_block.rb#L123) that already exists in your code is much better. I hope that I don’t have to explain more widely why it’s very bad to access directly instance variables anywhere in templates. :smirk_cat: