Smart format selection

[vokimon]

Let the library be smart and given some clues deduce which is the format and subformat to use. For instance:

• When no main format is specified, deduce it from the file name extension. Say from file.ogg, it could deduce you want to dump an ogg vorbis file.
• When no subformat is specified, deduce it from the main format. Say you want to dump a Riff Wave, it could choose 16bits Linear PCM subformat.