Fix -l, --geometry-count option

[omarkhan]

Fixes #7.

Steps to reproduce

1. Find a GeoJSON file with >1 feature
2. Split into separate files for each feature with geojsplit --geometry-count 1

Expected behavior

1 file per feature.

Actual behavior

2 files per feature.

Fix

The bug comes from here:

geojsplit/bin/geojsplit

Lines 201 to 202 in a4344c4

	geometries = [data];
	geometryCount = 0;

The geometries array has a single entry, but we're setting geometryCount to 0.

This fixes the bug by removing the geometryCount variable and using geometries.length instead.