Add a knife vault download command for downloading encrypted files

This is maybe an infrequent use case, but we occasionally need to create a local copy of an encrypted file (i.e., one created by passing the `--file` flag to `knife vault create`).  I had this written up as custom knife command for our organization, but it seems general enough that it could be included in chef-vault.  The idea is to run something like `knife vault download ssh_keys my_user ~/my_user.pem` and wind up with a `~/my_user.pem` file locally that contains the contents of the `my_user` data-bag item.