Add -min-file-size option

[jeabraham]

We have found an addition of -min-file-size option to be very useful, for hard linking, since small files could be identical by coincidence, and anyway hard linking small files doesn't save a lot of disk space. See description below.

How would you like me to submit the suggested code changes?

We found hardlinking to be problematic with small files, which could be identical
by coincidence. Since hardlinking files could cause problems, if one is hardlinking
to save space it makes sense to focus on the larger files, to have more bang for the buck.

We added a -min-file-size option. So, for example, if you find on OSX that
the combination of Apple Mail and GMail has bloated your drafts folder with many
identical copies of huge attachments, you may want to hardlink the identical
copies of the larger attachments together:

rdfind -makehardlinks true -min-file-size 40000 /Users/jabraham/Library/Mail/V6

[jeabraham]

Oh, it looks like maybe you added this already in 9ba54fb in October 2018?? I should merge in your latest, I guess.