Consider using rc over env for config

The `rc` module allows the user to store the config for `ipfs-deploy` either in the current project, as `env` does, or in standard locations in `~` or `/etc` as per https://github.com/dominictarr/rc#standards

For things like account credentials, it'd be reasonable to store them in one place for all sites you use ipfs-deploy for, rather than in each project.