Add conversion methods for other file formats

[sgraaf]

Add conversion methods (to and from) for other file formats, such as:

• GeoJSON
• Keyhole Markup Language (KML), possibly including KMZ

[sgraaf]

For GeoJSON, the mapping between GPX types and GeoJSON objects seems pretty straightforward:

• GPX wpt --> GeoJSON Point
• GPX rte --> GeoJSON LineString
• GPX trk --> GeoJSON MultiLineString

All packed into a GeoJSON FeatureCollection!

[sgraaf]

The GeoJSON format is described in RFC 7946.

JSON schemas (for validation purposes) can be found here: https://github.com/geojson/schema/tree/gh-pages