Preserve shared properties of merged arcs

It would be nice if `topojson-merge` also preserved shared properties like `topojson-group`.

Example: Swiss lakes are split into multiple features in the source shapefile, and I'd like to preserve the name property on the merged lake.

![lac-leman](https://cloud.githubusercontent.com/assets/70969/2622162/d23da502-bc92-11e3-954c-bc318e699a99.png)