Consider embedding this code to official mongodb codebase

[lromor]

Hi, first of all, really excellent project. Thank you for solving this problem, I honestly don't understand why this is not part of the official bson/mongodb package for go. It was really hard to find it and is my personal opinion it's a fundamental feature to perform updates on mongodb documents in a way that is natural using struct tags. I'm not affiliated in anyway with mongodb but, if you agree, we could give it a try.