dir2ogg should not overwrite files

[naw]

Executing dir2ogg on a dir with files file.m4a file.ogg ends with file.ogg being overwriten.

dir2ogg should display a warning about that file and refuse to overwrite it unless the user explicity asks to do so (e.g. with --force-overwrite)

[Julian Andres Klode (juliank)]

I wish I could do something useful here, but the fact is that I cannot ask the user about it interactively, and not overwriting it might cause issues too.