Set outfilesuffix attribute to extension of source file

Resolves #46 and improves #45 
> To allow the document to overrride, just add an `@' at the end of the value. That makes the attribute mutable.

I was thinking about the option page because the user can add custom attributes, so I need to add outfilesuffix before adding the custom attributes ?

 * The file extension is `.asc` so we add `outfilesuffix=.asc@`
 * User adds `outfilesuffix=.asciidoc` (in custom attributes option page)
 * The document override with `outfilesuffix=.ad`

Is that correct ?