Change default copy behaviour to --parents

[igungor]

Not 100% sure about this, adding this here to track the issue.

Currently s5cmd flattens source directory hiearchy while downloading (cp/mv). For example:

s3://bucket/obj1
s3://bucket/obj2
s3://bucket/01/obj2

s5cmd cp 's3://bucket/*' .

The default behaviour is to find each object recursively, and download them while ignoring all prefixes after the wildcard. Result:

$ ls 

obj1
obj2

There'll be only 1 obj2 but we're not very confident which one.

We should make --parents the default behaviour, and --flatten an option.

[jackwellsxyz]

I might also suggest making '-r' an alias for --parents, which would bring s5cmd cp more in line with standard UNIX cp behaviour.