* Create a new command/config subpackage.
This PR extracts the functions associated with loading and parsing
configs, and the DefaultTokenHelper, into a command/config subpackage,
just like TokenHelpers are in the command/token subpackage. The goal is
to allow other clients (in this case, the Vault and Nomad Terraform
providers, but in theory any client that wants to lean on Vault's
default behaviour) to reuse this logic and not drift from Vault, without
vendoring the entirety of Vault.
To retain backwards compatibility, I didn't remove any functions from
the command package; I just copied them into the command/config package,
and update the functions in the command package to call through to the
config package.
