Create clearer data structures for keep_message and keep_warning

[hadley]

Like the similar change for stop_on_error, I think this change makes it easier to reason about what's going on.

I think this also better captures my original motivation for adding the log_warnings argument; you want some way to say "please capture warnings while continuing to show them in the console.". I'm not sure about the field names here, so please let me know if you have better ideas.

[cderv]

The field names are clear to me. Others that come in mind are on_message$keep and on_message$muffle but that is not better. capture and silence are clear IMO.

The code reads much clearer.