Add configuration option to disable environment inheritance

[duijf]

Right now, vaultenv calls exec() and explicitly passes on the old environment so the process inherits this. Some people may not want this. We can add a CLI option to make this configurable, such as --no-pass-env.
Naming suggestions welcome.

Since there are currently no known cases where this might be useful, I'm waiting on at least one person to actually request this feature before implementing it. Please add a comment if you want this.

[arianvp]

Yes makes sense. Also if you have the case where you pass the vault token through VAULT_TOKEN you perhaps do not want to expose that to the running process