Create new values

It looks like the tool only lets you get/set existing values and not create new ones.

```
$ cat test.hcl
foo {
    bar = true
}

$ cat test.hcl | hclq set 'foo.baz' 'this is new'
foo {
  bar = true
}
```

I would really love to be able to do:
```
$ cat test.hcl | hclq set 'foo.baz' 'this is new'
foo {
  bar = true
  baz = 'this is new'
}
```
