Extend constructor to allow for async args

[dy]

Similar to Image:

Audio('./x.wav', (err, a) => {})
Audio(blob).on('load', a => {})
Audio(file).then(a => {})

1. That would allow to get rid of / hide .decode, .load methods.
2. Audio w/o content loaded is in temporary state of empty buffer, but available already.
3. That complicates Promise interface Audio(url).then(a => {}): audio is thenable.
4. That mixes up sync/async pattern: some audio sources are available synchronously, like buffers, others only in async way.

Otherwise it is not clear the natural way of creating blobs: Audio.load(blob),Audio.decode(blob) or Audio.from(blob)?