Warn if no files are checked

[dingens]

When you just run duperemove it won't run normally, but display the usage notes instead.

However, when you run duperemove -h or -r, it seems to run normally, but actually checks nothing at all (because no files/directories are given). The output is very misleading then:

$ duperemove -h
Gathering file list...
No dedupe candidates found.

I suggest to either

• assume . as argument if no directories are listed (as does ls for example), or
• when duperemove is called in such a nonsensical way, warn the user. Or just present the usage dialog, like when it is run without any arguments.

[JackSlateur]

Thank you @dingens for the report!