Use --out-file break the links to home about asciidoctor-multipage

[owenh000]

Hi @aplatanado, thanks for the report. Here are some brainstorming ideas:

• First, the extension should detect when --out-file is used, if possible. If it's not possible, then we can only document this option as unsupported.
• If --out-file can be detected but is not supported, the extension should fail without generating output.
• Ideally the extension would simply use the --out-file value in place of the node ID when generating links to the main page. (This behavior would be unrelated to whether or not the output filename has an extension.) It would also be necessary to make sure the relative paths work between an --out-file path and the --destination-dir (and fail with an error if that is impossible).

Also, this is related to issue #21.