Adding "Trim unallocated space" for f2fs

f2fs is quite a popular filesystem for SD cards. Could you add the efficiency of trimming unallocted space for f2fs, like ext-type filesystems?

Hi @gleng interesting idea, would you be able to open an issue for this on the etcher repo: GitHub - balena-io/etcher: Flash OS images to SD cards & USB drives, safely and easily.