Allow passing the file format as a property.

Currently you detect the audio file format from the file extension, which makes a lot of sense. I'd like to be able to also load sounds that don't follow that convention by specifying the format. For example, I make an API request that returns an mp3 file however it does not end with .mp3 in the url. Such as:

    var sound = new Howl({
      urls: ['http://webservice/awesome?query=hello'],
      format: 'mp3'
    });