Do not try to open music database for basic commands (--help, --version, etc)

Current version (`1.3.11`) tries to open database file even for purely informational commands.
In cases when database file resides on some external and unmounted drive, this makes it impossible to see output of `beet --help` or `beet --version`, because user gets an error instead:

```
error: database file /path/to/musiclibrary.blb could not be opened
```

Would be nice to have `beets` provide this info without probing for the DB file.