Feature/load gpx track

[matzepan]

Added the ability to load tracks (kml, gpx, geojson, json) using leaflet-filelayer and display them as new Overlay layer.
Addresses: #30

[bagage]

Hey @matzepan, welcome aboard! Thanks for your contribution, it is one of the most asked-for feature. From what I've seen it's looking good, albeit a minor issue: it looks like tracks are declared as interactive layers but it seems they are not:

Good work!

[matzepan]

@bagage this is also a feature that I wanted badly.
I changed the loaded track to be not interactive.
Thank you for your feedback.

[Phyks]

What about keeping the loaded track in a URL parameter for easy sharing of the full view? For instance by encoding the polyline with mapbox/polyline in a track (tracks?) GET parameter similarly to #233 (which I should finish by the way)?

[matzepan]

@Phyks I don't know if it makes sense to put these into the URL. Especially if this means blowing up the URL length.

[Phyks]

It makes sense to have it to easy share the view with someone else / another computer. But indeed it probably does not make sense to have it in the URL and risk blowing the URL length.

Maybe BRouter-web should have something like Overpass to shorten links and share easily (including custom profiles, etc).

[nrenner]

Great contribution, thanks!