adds stem section to user manual

- stem attribute, inline macro, stem block
- latexmath and asciimath interpreters
- documents asciidoctor/asciidoctor#492, asciidoctor/asciidoctor#760,
  asciidoctor/asciidoctor#1005
Merge pull request #228 from graphitefriction/1-5-docs

adds stem section to user manual