Support dialects

[jxxcarlson]

Asciidoctor-latex now processes three "dialects". See the below for how these are invoked, and see asciidoctor.shoobox.io for a description of the dialects. Can we modifiy the 'latex' branch of 'asciidioctor-doctest' so as to be able to test each of the three dialects separately?

I tried this but it did not work:

# html_test.rb
class HtmlTest < DocTest::Test
  converter_opts backend_name: 'html', dialect: 'latex'
  generate_tests! DocTest::HTML::ExamplesSuite
end

new syntax with switches for three 'dialects' See below.

# Invoke asciidoctor-latex for LaTeX output by
#
#   asciidoctor-latex -a dialect=asciidoc foo.adoc
#   asciidoctor-latex -a dialect=manuscript foo.adoc
#   asciidoctor-latex -a dialect=latex foo.adoc
#
# Be sure to use XeLaTex for tex'ing
# For html output, use
#
#   asciidoctor-latex -a dialect=asciidoc foo.adoc -b html
#   asciidoctor-latex -a dialect=manuscript foo.adoc -b html
#   asciidoctor-latex -a dialect=latex foo.adoc -b html
#
# The above are *source file* options for dialects of asciidoc:
#
#   asciidoc
#   asciidoc-manuscript
#   asciidoc-latex
#
# Ruby API to convert to HTML:
#
# Use for example
#
#        Asciidoctor.convert str, { 'dialect' => 'latex' }
#