utf8 decoding failed on file_info.comments

Hello there,

I got this FLAC audio that displays these information when I run `soxi -a audiofile.flac` :

```
$ soxi -a 1989_7240_000022.flac
artist=Giacomo Leopardi
DESCRIPTION=https://archive.org/details/
genre=Speech
title=13 - La sera del d� di festa
album=Canti
TRACKNUMBER=13
encoder=Lavf57.83.100
```

When I use `sox.file_info.comments()` I got an `UnicodeDecodeError` at

`shell_output = shell_output.decode("utf-8")`

in core.py, line 155

Right now I just catch that errror and call soxi as a subprocess and then decode it with

`.decode("utf-8", "ignore")`

but also a "replace" flag could be useful and can be given to the `shell_output.decode` line

https://github.com/rabitt/pysox/blob/7e0891a40ad4e29e2a67e6abf826cd048377231c/sox/core.py#L162

Start in sox/core.py around the shell_output.decode("utf-8") call at line 155, using the reported FLAC metadata to reproduce the UnicodeDecodeError. Review the proposed decode error handling and make the expected behavior clear for invalid bytes, then verify that sox.file_info.comments() handles this file successfully.

Written by the indexing model from the issue text.