Consider adding StackExchange/blackbox workflow

[josegonzalez]

We're currently using StackExchange/blackbox in a few internal repositories, and would like to switch those to using agilebits/sm. Would you consider a pull request to add such a workflow to this project?

A few things we'd add:

• keeping a manifest of encrypted files (.sm/manifest)
• registering new files to this manifest
• bulk file decryption
• shredding all decrypted files
• key rotation
• editing an encrypted file and re-encrypting it when done
• adding a "diff"

If the above doesn't seem palatable, I'd be happy to create a new project and start implementing the above featureset on my own. Thoughts?