Set outfilesuffix attribute to extension of source file

Resolves #46 and improves #45 
Correct. We can't include the trailing '@' or else Asciidoctor will overwrite the attribute when the backend is setup.

The main purpose of the outfilesuffix is to automatically determine the file extension of the file to write after converting. There's a default file extension for each backend. As you would expect, the default file extension for the html5 backend is `.html`. However, when using the live preview, we are viewing the AsciiDoc files directly. Therefore, we want inter-document cross references to resolve to the AsciiDoc files instead of the generated HTML files as Asciidoctor does by default. This allows the viewer to jump from one AsciiDoc file to the next through inter-document cross references.

Of course, we are assuming that authors are using the same file extension for all related AsciiDoc files, but that's a necessary assumption since we can't read minds :)