--require option should be optional for "official" backend

[ggrossetie]

For instance if the package asciidoctor-reveal.js is installed then a user could use the following command:

asciidoctorjs -b revealjs source.adoc -o dest.html

Shorthand for:

asciidoctorjs -r asciidoctor-reveal.js -b revealjs source.adoc -o dest.html

In other words, it means that the CLI knows a set of "official" backends:

• pdf
• revealjs
• bespoke
• other ?