Echo JS 0.11.0

Good question! Yaml could absolutely work, but I found it too verbose. The advantage of a custom syntax is that the declaration is very similar to a .env file (or output from `env`). With yaml it would have to look something like:

```
VAR:
  type: String
  default: VOO
  # no obvious place for comments
```

To me, at least, a terse and intuitive syntax is more important than <insert reason for using yaml here>. However the internal representation looks exactly like that, so in theory it would be possible to support yaml/json too.