Infer output file format type when not specified

[sgreenbury]

As mentioned in #22, allow inference of the output file format given a provided output file name (e.g. using the suffix).

[penelopeysm]

An interface that makes sense to me (imo):

--output-file	--output-format	Outcome
<empty>	<empty>	prints csv (?) to stdout
<empty>	geojson	prints geojson to stdout
a.csv	<empty>	outputs csv to a.csv
a.geojson	<empty>	outputs geojson to a.geojson
a.geojson	geojson	outputs geojson to a.geojson
a.geojson	csv	outputs csv to a.geojson with a warning