Beets shouldn't colorize (at least by default) if output is to a file or pipe rather than a tty.

### Problem

This will output bad files to "bad-mp3.txt" but will also include ansi escape sequences which makes the file hard to read.

```sh
beet bad format:mp3 | tee bad-mp3.txt
```

ideally, beets shouldn't colorize (at least by default) if output is to a file or pipe rather than a tty.

