keys command should accept keys from standard input

[dmolesUC]

Passing - as a file argument should read keys from standard input, e.g.

cat mykeylist.txt | cos keys s3://uc3-s3mrt5001-stg/ \
  -e https://s3-us-west-2.amazonaws.com/ \
  --file -

If we want to do this right, we should probably put KeyList take a passed-in function and iterate over its own keys rather than having it expose a []string. This poses problems for KeyList.Count, but we don't use KeyList.Count much except for documentation and for final output.