Handle multiple CSV paths.

lib/reader.js
	-Bundle multiple files into a single data-stream created by
	`createReader()`.
Handle multiple OSM paths.

lib/reader.js
	-Add support for multiple OSM files, by aggregating multiple
	Osmium data streams into one.
Handle multiple shp paths.

lib/reader.js
	-Handle multiple shapefile paths.
Refactor createReader() into multiple functions.

lib/reader.js
	-Refactor `createReader()`, partitioning its now sprawling
	`switch` statement into multiple logical component functions:
	`createReaderCsv()`, `createReaderShp()`, and
	`createReaderOsm()`.
Add documentation to new lib/reader functions.

lib/reader.js
	-Add documentation comments to all of the functions introduced
	by b4b4e2d.
Update README doc to mention multiple paths.

README.md
	-Update the README with documentation about multiple file paths
	in reader objects.
Allow multiple paths in reader objects.

lib/reader.js
	-Adapt `lib/reader` to allow multiple paths per `reader`
	configuration object, in case multiple files should be imported
	using the same pipeline and remapped with the same `mapper`.