Cannot load audio file by extension

[justindujardin]

AudioResource is registered with the ResourceManager in a way that requires no extension to be specified when loading audio files. It's also perfectly legit to want to specify the exact file, and avoid network requests on the other audio extensions.

Add support for loading an audio file with it's full extension, e.g. "myfile.mp3" or "myfile.ogg", and continue to allow specifying without an extension, e.g. "myfile".