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.
I'm pretty sure you can just do `Asciidoctor::VERSION < '1.5.2'` here, or are there gaps because of pre-release versions and the like?