Set outfilesuffix attribute to extension of source file

Resolves #46 and improves #45 
I don't think you have to be so detailed with getting the file extension. In other words, I don't think we should support double or triple file extensions. Just take everything after the last `.` character.

To allow the document to overrride, just add an `@' at the end of the value. That makes the attribute mutable.