Add Support for Multiselect

This PR adds support for multiselect, which is the ability to grab a subset of keys from a dictionary:

```
$ echo '{"foo": {"bar": 1, "baz": 2, "qux": 3}}' | jp 'foo.{bar,baz}'
{
    "baz": 2,
    "bar": 1
}
```

Any identifier can be an unbranched expression so you can have jmespath expressions like this: https://github.com/boto/jmespath/compare/multiselect?expand=1#diff-a7ca2d90b652a200d5572caabd204f88R94

Compliance tests: https://github.com/boto/jmespath/compare/multiselect?expand=1#diff-a7ca2d90b652a200d5572caabd204f88R1

cc @garnaat @danielgtaylor