Add bit depth attribute

[robbevp]

It would be great if this library would return the bit depth of the audio file. Since you already use bits_per_sample internally, it wouldn't make a big change to rename this to bit_depth and expose the property. I got this to work locally for both flac and wav.

However, files with lossy compression don't have a bit depth. I'd propose to just leave this nil.
The only encoding I'm unclear on is ASF since WMA can be both lossy and lossless.

It be happy to work on a PR for this, but wanted to check-in first.

[aidewoode]

Hi, I think we can add the bit_depth attribute, as you said, add this attribute is not hard. I have checked the asf tag. We can get bit_depth info on stream_properties_object. PR is always welcome If you want to work on this. And if you have any question about it, you can contact me.

Thanks.