add PoC cmd converting HCL to JSON

This adds an `hcl2json` command which reads HCL from STDIN and writes indented JSON to STDOUT.

At the moment it is not friendly at all, and probably is only useful for people who are testing HCL code changes. Before merging, it should be a friendly tool to allow people who :heart: HCL to use it in environments where importing this package is unwieldy.

Some things it should probably do:
- [ ] `hcl2json`, `hcl2json -h` should print usage
- [ ] `hcl2json foo.hcl` should read from `foo.hcl`
- [ ] `hcl2json -` should read from STDIN