[Feature] safe all-in-one invocation

[jhunt]

I would really like to be able to do this:

safe locally -- \
  x509 issue --ca secret/ca -n my.ca --ttl 1y \
  x509 issue --signed-by secret/ca secret/cert -n my.cert \
  export --to-disk

There's a bit to unpack here:

1. safe locally should do what safe local does, but (a) always use memory-backing, (b) not muck about with ~/.saferc, (c) not block and (d) continue parsing commands.
2. safe export --to-disk should export the contents of the given tree (or secret/) to disk, turning path components into directories, and keys into files containing their respective values.

This would finally let me use safe x509 to build out the complicated mess of mutual TLS certificates that BOSH and Cloud Foundry require without spinning up a Vault and having to multi-process. safe local works great for interactive use, but in a one-shot scripting scenario like this, leaves much to be desired.