new feature: delete only if new file size is smaller

[mikelesagitter]

Hi, I don't know how to propose a feature so I create a bug :)

When I batch convert, at the end of conversion I have to check either or not each new file is smaller then the older. Normally I would use the --delete option but I dont want a file to be bigger after conversion!

It would be nice to have a sort of --deleteifsmaller allowing to delete old file only if the new is smallest :)

thanks