Add file size based filter.

[Pratik32]

I wish to add a new filter to the utility. based on file size.
i.e the command will only work on those files that satisfy given file size constraint.
e.g
$ nmly reverse -d test/ -s -6G
Above command will only reverse those files which are under size 6GB.

$ nmly reverse -d test/ -s +15M
Above command will only reverse those files which are above or equal to 15 M size.