Preserve GPX metadata and trkpt extensions in converted geojson

[opoto]

In addition to the heart rate extension that is already converted, this commits stores metadata and all GPX trkpt extensions into the generated geojson, so that this data is not lost and can be processed by the application. The corresponding xmlns are also stored.

[andrewharvey]

@opoto you might want to checkout https://github.com/tmcw/togeojson as a more maintained fork

[opoto]

@opoto you might want to checkout https://github.com/tmcw/togeojson as a more maintained fork

Thanks for the notice. I will consider migrating, but I made significant updates in current version which I might want to preserve.