Implement a sign-many command for better performance

[tomasr]

This is a proposed potential way to add the functionality to (sign multiple files at a time)[https://github.com//issues/34] when using the local client tool instead of the separate server process.

May not be the best way to go about it, but seemed like the simpler option, as it's very similar to the existing sign command but without support for stdin for input.

If you're signing multiple packages of the same type using an HSM or Cloud-backed token, the performance gains are substantial compared to doing it one at a time.