Add "import" support for vaulted load when vault exists

[thedannywahl]

It'd be nice if vaulted load could import and overwrite/merge passed values when the target vault already exists instead of nuking the existing vault and recreating it. Here's my use case:

I have a front end that allows users to configure the contents of the vault. Previously this modified a dot file, now it modifies the vault. In order to maintain this functionality I have to do the following:

1. use vaulted dump to dump the vault to memory (prompt user for vault password)
2. prompt user for keys
3. use jq to insert keys into vault JSON in memory
4. use vault load to recreate the vault. (prompt user to create vault password)

obviously this leaves some room for error as it might not be obvious to the user that the vault is being recreated, and at best it's a lot of password prompts. If load supported import/merge the workflow would look like this:

1. prompt user for keys
2. insert keys into vault with vault load (prompt user for vault password)

For my specific case I don't care if an existing key in a vault is overwritten with a new value (as that's what's happening now anyways).