support not adding a newline for empty hashes when pretty_generating

currently, `pretty_generate` will add a newline to the output if there is a hash or array that is empty:
```json
{
  "blah":"bar",
  "foo": {
  },
  "baz": [
  ]
}
```

this is to add an optional argument to not generate that newline. e.g.
```json
{
  "blah":"bar",
  "foo": {},
  "baz": []
}
```