pass context properties along to nlog

Pass all the context properties along to nlog, which can then be included in the [nlog output](https://github.com/NLog/NLog/wiki/EventContext-layout-renderer), e.g. `${event-context:item=somePropertyItem}`.
This seemed like a simple way to get scalars without the quotes, etc. that existing nlog users would not expect. If there's a better approach, please school me!