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.:

django-filebrowser/filebrowser/management/commands/fb_version_generate.py

Line 70 in 6686ffe

versionobject = fileobject.version_generate(version) # FIXME force?

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