Add a `--force` flag to `python manage.py fb_version_generate`

[andylolz]

I notice there are a couple of references to a --force flag in the fb_version_generate code, e.g.: https://github.com/sehmaschine/django-filebrowser/blob/6686ffeb00bac1779539992d7bea6253522606f2/filebrowser/management/commands/fb_version_generate.py#L70

This would be a useful addition. Would you be happy to accept a pull request for that? Thanks