Load Vault token from file

Vaultenv is a fantastic tool and has really helped my organisation simplify our developer workflow. One additional feature that I’d love is the ability to load the Vault token from a file.

We currently use Docker and distribute the token to our swarm nodes via the `VAULT_TOKEN` environment variable. The problem is that if an unauthorised user gained access to a node they could simple do `docker inspect` to retrieve the token.

If Vaultenv could load the token from a file, e.g. via a path specified with the `VAULTENV_TOKEN_FILE` environment variable, we could then use Docker Secrets to distribute the token to the node, which would be more secure.