Implement "optional object identifier index" filter

One of the missing features today is the "optional object identifier index" that jq provides out of the box.

Below are examples using `jq` today but `tq` needs the equivelent support for TOML files.
```
{"foo": 42, "bar": "less interesting data"} | jq '.foo?' // 42
{"notfoo": true, "alsonotfoo": false} | jq '.foo?' // null
```
