Allow configuring whether disused/abandoned venues are removed

[orangejulius]

Currently, our popularity mapper from #493/ #531 also causes any venues with low popularity (usually those marked as disused or abandoned in OSM) to be removed, and not imported into Pelias:

openstreetmap/stream/popularity_mapper.js

Lines 202 to 203 in 02e4fd7

	// discard places with a negative popularity
	else if( popularity < 0 ){ return next(); }

This might be a reasonable default, but it's also pretty significant. There might be a lot of notable places that fall into those categories. We should look at adding a configuration option to enable or disable removal of those venues.

[orangejulius]

This was fixed in #539: by default we will now continue to import these venues, but there is a configuration option to remove them if desired.