rs_cover, rs_download, and rs_rasterize should accept multiple GEOJSON files

With the introduction of the batching feature extraction (#148), the inputs to rs_cover, rs_download, and rs_rasterize are likely going to be multiple GEOJSON files. It would be nice if these tools accepted multiple files as inputs. My current workflow is as follows:

CURR_DIR=$(shell pwd)

# Download full extract
download-extract:
	wget --limit-rate=1M $(PBF_DOWNLOAD_FILE) -O $(CURR_DIR)/data/extract.osm.pbf

# Cut out the area we are interested in.
cut-extract:
	docker run --rm -v $(CURR_DIR)/data:/osmium/data asymmetric/osmium-tool extract --bbox $(BBOX) data/extract.osm.pbf --output data/map.osm.pbf

# Extract geometries from an OpenStreetMap base map
extract-geometries:
	docker run -it --rm -v $(CURR_DIR)/data:/data --ipc=host --network=host mapbox/robosat:latest-cpu extract --type building /data/map.osm.pbf /data/buildings.geojson

# Generate all Slippy Map tiles which have buildings in them
GEOJSON = $(CURR_DIR)/data
generate-slippy-maptiles: $(GEOJSON)/*.geojson
	@for file in $^ ; do \
	 	bn=$$(basename $$file);\
		docker run -d -it --rm -v $(CURR_DIR)/data:/data --ipc=host --network=host mapbox/robosat:latest-cpu cover --zoom $(ZOOM) /data/$$bn /data/$$bn.tiles; \
	done

Start by tracing the CLI entry points for rs_cover, rs_download, and rs_rasterize and review how the batching feature from issue #148 supplies GEOJSON inputs. Define the multiple-file input behavior across these three tools, then verify that the tools process the listed files and produce the expected outputs.