Passthrough configuration

In some situations, such as build and ad-hoc testing environments, developers may which to validate containers and functionality without going through the overhead of setting up or configuring vault (or consul). Without it being an optional capability, some developers would *rather not use it at all*

`envconsul` makes a great container `entrypoint` and hits that niche perfectly, however teams I work with are refusing to add it to their images because they can no longer play with their containers in development and testing sandboxes without Vault/Consul setup overhead (where they might manually specify the environment variables envconsul is expected to provide).

Can a "passthrough" mode be established, where if a specific config flag is set, it simply execs the primary container process without talking to consul or vault? This allows me to drive envconsul adoption by ensuring it is added to the entrypoint of the containers for teams i support.

Thanks!

### Configuration

```
{
passthrough = true
}
```

Dockerfile
```
ENTRYPOINT ["/path/to/envconsul", "-config", "/etc/envconsul/config.hcl", "--"]
```

Command (example, Docker)
```
docker run -v ./config.hcl:/etc/envconsul/config.hcl myimage:latest
```