Asciidoctor: Add option to include revnumber in output filename

[mojavelinux]

It would be helpful to allow the revnumber to be automatically included in the name of the output file. This saves the user from having to remember to use the -o flag to specify a file name and align it with the revnumber.

If there's an easy way to do this without built-in support, then we should simply document it.

[mojavelinux]

We'll need an option or attribute for this. I think an option makes the most sense since this is really something that is controlled at the level of the -o flag. Perhaps --version-out-file?

[mojavelinux]

I think an option is definitely the right choice here as the output file is something that is controlled by the API.

[mojavelinux]

Technically this can be done by hacking the outfilesuffix in the document header:

:outfilesuffix: -{revnumber}{outfilesuffix}

However, this can have side effects, so it's still better to have an option.