default to reasonable buffer size

[springmeyer]

The default 256 buffer size is overly large. We should default to something much smaller, like 5 (which is the default in tippecanoe). Most Mapnik vector tile renders will and should override this value in the mapnik XML by setting the buffer-size on the Layer. This value is then ignored, which has always been the case for tilelive-omnivore usage of tilelive-bridge. But nevertheless this default is too large.

refs mapbox/tilelive-omnivore#45

Refs #87

[springmeyer]

won't get around to testing this, so closing.