Fix S to allow for other SearchResults classes

[willkg]

If you wanted to write your own SearchResults class and use it,
you'd have to override non-trivial and private innards of S.

This makes that possible.

Also, I changed decorate_with_metadata so that it's not marked as
"private". That's a helpful function.

r?

[willkg]

My big use case is that I want to create a SearchResults class that holds a list of FauxObject classes which have the data that ES returns for the result, plus the metadata (highlight, score, etc), plus methods that help me use that.

[willkg]

@robhudson Can you take a look at this when you have a chance?

[willkg]

I'm going to nix this. We've got a better idea, so I'll work on that instead.