allow config volume

This provides the ability to mount a docker data volume (i.e. `-v /my/consul/config:/config`), but in a way that prevents overwritten the initial config file.  Any data placed within the config-dir should overload the default config settings.

This provides an indirect 'fix' for #25, as well as providing another way to share things like CA certs (in my env, a systemd task pulls them down from an encrypted s3 bucket to place on the server; I could create a docker image but I would rather not...)

thanks.  fantastic project, btw.