Add support for specifying multiple files on the command line and preserve file paths

[oxsdd]

In other words, work like tar.

For packing:

ipfs-car --pack a1/b1/c1.dat a2/b2/c2.dat f1.dat --output out.car

This avoids the need to link disparate files into a common root directory.

On unpacking:

ipfs-car --unpack out.car

would result in the following in the current working directory:

a1/b1/c1.dat
a2/b2/c2.dat
f1.dat

This avoids the need to track the full file paths separately.