Support send2trash appropriately

One of the big flaws that I dislike about fdupes is the irrevocable nature. It would be nice to send files to the trash. Fortunately, there is already a nice library for this purpose, in send2trash.

By all accounts, it would just take a few short lines to make this happen. I have already done so with my own copy of fast dupes, but I figured that you would want to do it your way, if you agree to its inclusion.

Life has left me backlogged on fixing fastdupes, but I'll consider it once more pressing things are resolved.

It'll have to be some kind of optional thing, though, because I'm the primary target user for FastDupes and I've been building an "either clear the trash immediately or turn it off" habit ever since Windows 95. (When I run fastdupes, it's because I'm running low on space and need it immediately. If I want safety, I'll prioritize implementing "hardlink, don't delete".)