Decoder input mp3 info

exposed mpg123_info to get information about the input mp3 format (bitrate, samplerate, etc.)

I just added a public ```.info()``` method, I wasn't sure but it might be useful to include that info when triggering the ```format``` event.

one use case I wrote this for is transcoding, I want to transcode from quality A to B, but I want to passthrough (omit the transcoding) if A === B.