Added verbose option that writes diagnostic messages to stderr

This implements part of #14.

I also have code to check/repair invalid polygons (edge crossings, duplicated nodes, etc.) but that seems better suited for a separate GeoJSON lint tool rather than as a part of format conversion. 
In this context, `tag_object` can either ba a multipolygon-relation or an outer way of a *simple* multipolygon. Only printing the id could be ambiguous. A similar thing can happen in the case of `geometry="center"` output from Overpass-API 0.50 (see the [neogeometries](https://github.com/tyrasd/osmtogeojson/commit/f68658f3cd8c5b0b4be03506d6f0a9ee33988a7e#diff-168726dbe96b3ce427e7fedce31bb0bcR275) branch) where a POI may be a node, way or relation.

Maybe it is better to print all id's including the corresponding OSM data type. For example: `Multipolygon relation/123 ignored …` or `Multipolygon way/321 ignored …`. And for consistency we should probably use this scheme for all warnings, I would say.