Fixes #1237. -s CLI option should be changed to -e to align with Asci…

…idoctor

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Kind of change

[x] Bug fix
[ ] New non-breaking feature
[ ] New breaking feature
[ ] Documentation update
[ ] Build improvement

Description

What is the goal of this pull request?

Fix #1237 and change the command line option -e so that it creates an embedded document instead of selecting eRuby.

How does it achieve that?

Add a new cli option --embedded with the short key -e. Change --eruby so that it doesn't have any short key.

Are there any alternative ways to implement this?

Likely not.

Are there any implications of this pull request? Anything a user must know?

I also removed the explicit mentionings of the defaults in the argument descriptions, since jcommander also shows the defaults, e.g.,:

    -e, --embedded
      suppress enclosing document structure and output an embedded document
      Default: false
    --eruby
      specify eRuby implementation to render built-in templates: [erb, erubis]
      Default: erb

Issue

If this PR fixes an open issue, please add a line of the form:

Fixes #1237

asciidoctor / asciidoctorj