Enable option to always use files' paths with play

When using `play` with the `-a` option, the m3u output lists folders, which is incompatible with some players. This PR adds a possibility to always use filenames instead of folders. Since I'm not sure if there are use cases for strictly using folders, I added a config option, but IMO this should be the default behavior, so that's why I'm opening a PR to get some feedback.