New "Policy" model to configure how different document members are handled via Linq

[jeremydmiller]

Breaking this off from #751. This is specifically to control how JsonLocatorField's treat different types. There's some hard coded logic in there now, but maybe we could break that out to an extension point that also allows users to add new behavior.

Thinking that this would be valuable for a future "Marten.FSharp" package and possibly for the work @jarroda has done on camel cased JSON storage.