Allow sdk control over initial visibility for buildings, minor roads #243

In the current design we show things like buildings at zoom 13 and minor roads at zoom 12. This looks great, but sometimes has a performance impact. Let's add a SDK config to throttle back when some features are shown.

This probably involves making use of layer enabled properties for buildings (added in Tangram v0.12, was visible in earlier releases) and possibly also looking at individual feature min_zoom with an offset (like for roads).