add loadArrayBuffer method

I was trying to use `wavesurfer.load(url)` to load a blob (ur)l in firefox, eg. 'blob:7c2853fc-41c1-4480-b078-65c9e5b0b5b3' but couldn't get it to work with xhr, so added a `loadArrayBuffer` that allows you to load a `Blob` or `File` object. This allows you to load local audio data recorded from a mic for example.