Set outfilesuffix attribute to extension of source file

Resolves #46 and improves #45 
> In reality, I can't imagine anyone needing to set outfilesuffix in the Chrome extension, and using the file extension of the current file is the most sane default.

I agree but I'm a little bit confused... you are saying that I need to set the outfilesuffix using the file extension of the current file without @ ? Otherwise Asciidoctor will overwrite the value ?