Add a command-line option for automatically hardlinking dupes

[ssokolow]

Issue #5 proposes a --noninteractive option which causes --delete to fall back to keeping all entries if no --prefer option matches.

There should also be some similar mechanism for falling back to hardlinking but I'll need to think about how to cleanly factor the API so everything's nice and orthogonal without constraining the functionality.

[e3krisztian]

+1 for hardlinking, would be very useful with --min-size