Allow "unsafe" paths which contain directories.

### Proposed solution

I want to be able to use the `convert` plugin to transcode (for example):
```
/mnt/foo/Music/albums/Album Artist/Album Name/
```
to
```
/mnt/bar/Music/albums/Album Artist/Album Name/
```

It's driving me mad that beets escapes every `os.pathsep` to `_` or `__` resulting in:
```
/mnt/bar/Music_albums_Album Artist_Album Name_01 baz.ogg
```

I tried overriding `sanitize_path()` in `beets/util/__init__.py` to simply:
```
def sanitize_path(path):
return path
```
But `destination` in `beets/library.py` appears to be applying sanitization to each field of the path.

Can a config flag like `unsafe_paths: true` be added for users who know what they're doing which prevents this behaviour?

I'd be happy to leave the flag undocumented if you're especially concerned?

I can _probably_ make the PR myself but I won't start unless there is a will to merge it.

Start with the convert plugin and inspect sanitize_path() in beets/util/__init__.py and destination in beets/library.py, where the issue reports that path fields are sanitized. Trace how configured destinations are assembled, then define the unsafe_paths behavior so directory separators can be preserved without changing the default; verify the example destination and add coverage if the relevant tests are found.

Written by the indexing model from the issue text.