add function to_object to parse nested json string

[litao09h]

[joerodriguez]

I like this solution, though unfortunately it's not part of the jmespath specification. I did however find that to_array should be giving pretty much the same behavior as your to_object function. This is not yet the case in go-jmespath, so I've opened https://github.com/jmespath/go-jmespath/pull/93 to fix the issue.

This PR can be closed in favor of https://github.com/jmespath/go-jmespath/pull/93