:detect should be a setting for filename and not extension

[Sharpie]

Right now, setting a value of :detect on the config attribute :extension will cause PEBuild to construct a filename based on the version and Guest OS type. However, in some situations, it is useful to have the filename automatically built but be able to specify an explicit extension.

Thus, it would make sense to have :detect as a value set on filename which frees extension up to contribute information to the detection process.

[MikaelSmith]

This sounds useful, since I ofter have a tar-file rather than tar.gz, and it's not correctly detected.