Allow filtering of OSM data

[miccolis]

When loading osm data with loadOSMExtract we should support a way of filtering what information we actually store. For example, it should be possible to only make speed limit or road class information available.

[danpat]

There are two things we need to make configurable at startup:

1. A filter on the ways we actually want in the index. Currently, this is a hard-coded list here: https://github.com/mapbox/route-annotator/blob/master/src/extractor.cpp#L100
2. A filter on which tags from those ways to index. Currently, we index everything on ways that pass (1).