Switching to unconstrained formatting sequences

[skalee]

AsciiDoc supports two types of formatting sequences:

• constrained ones, e.g. _text_
• unconstrained ones, e.g. __text__

We use the former but I guess we should switch to the latter, as they can be used inside words.

Documentation:

• https://docs.asciidoctor.org/asciidoc/latest/text/#constrained
• https://docs.asciidoctor.org/asciidoc/latest/text/#unconstrained
• https://docs.asciidoctor.org/asciidoc/latest/text/troubleshoot-unconstrained-formatting/

Also, it may help in metanorma/coradoc#92.

[opoudjis]

For the purposes of this gem: agreed.

[hmdne]

This is a duplicate of #43 I think. We handle both cases to produce the simplest correct AsciiDoc possible. All examples provided in that issue are in the test suite.