Update leaflet.heightgraph to fix marker hiding & more

[tbsmark86]

fixes #408 and similar bug in the graph itself

includes nrenner/Leaflet.Heightgraph#1 to display selection stats

Of course there is no need to switch the heightgraph repo to me. If instead you like to pull the changes into your heightgraph repo and let the package.json point there.

[tbsmark86]

After diving into the height graph code i played around with it some more and also worked on #361 and maybe #416 - meaning syncing the color on the map with that of the graph. Downside is that is very colorful now because of course real road are rarely steady climbing:

• Someone more clever may be able to get a better smoothing int the elevation data
• As you can see in the screenshot i also changed the 'selection' behaviour to a 'zoom in' on the graph
• And as a bouns i've added surface and highway to the graph's tooltip

What you guy's are thinking about this?

There are further directions we could take this

• syncing the route summary with the selection of the graph (instead of showing those values in the tooltip) (Feature Request: Enhance height diagram indicator with information about marked distance/elevation gain #434)
• add an additional line on top to show the surface coloring
• or: always show the same color as the hotline (unless hotline shows nothing then show gradient)
• sync the tooltip of graph with the detailed table display highlight
  Any more thoughts on these?