Use HCL

With 8e76d98 , we began using HCL for IO configuration. Pros HCL has great support in Go. HCL is much more readable than YAML/JSON. IO configuration is beautiful. Although IO could be configured interactively, it needed a mechanism for automated configuration. HCL works for that. Although IO's HCL is IO-specific, the general structure of HCL is familiar to users of popular Hashicorp products (Terraform, Nomad, etc). Cons There aren't offical support libraries for HCL in languages other than Go. HCL is more effort to read and write than YAML.