Merge pull request #227 from graphitefriction/1-5-docs

udates include dir and video for 1.5
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