Add copy file command to warcli

[m3dwards]

It would be useful to copy files from and to tanks.

I propose that we follow the pattern of scp, docker cp and kubectl cp by putting tank: in front of the path that for the tank.

Examples:

Copy from tank - warcli cp 0000001:/etc/hosts /tmp
Copt to tank - warcli cp /tmp 0000001:/etc/hosts